diff --git a/gio/gcredentials.c b/gio/gcredentials.c
index 9423f9ef9..f7393d4c8 100644
--- a/gio/gcredentials.c
+++ b/gio/gcredentials.c
@@ -196,7 +196,7 @@ g_credentials_init (GCredentials *credentials)
  * Creates a new #GCredentials object with credentials matching the
  * the current process.
  *
- * Returns: A #GCredentials. Free with g_object_unref().
+ * Returns: (transfer full): A #GCredentials. Free with g_object_unref().
  *
  * Since: 2.26
  */
@@ -216,7 +216,7 @@ g_credentials_new (void)
  * that can be used in logging and debug messages. The format of the
  * returned string may change in future GLib release.
  *
- * Returns: A string that should be freed with g_free().
+ * Returns: (transfer full): A string that should be freed with g_free().
  *
  * Since: 2.26
  */
@@ -435,10 +435,10 @@ credentials_native_type_check (GCredentialsType  requested_type,
  * logged) to use this method if there is no #GCredentials support for
  * the OS or if @native_type isn't supported by the OS.
  *
- * Returns: The pointer to native credentials or %NULL if the
- * operation there is no #GCredentials support for the OS or if
- * @native_type isn't supported by the OS. Do not free the returned
- * data, it is owned by @credentials.
+ * Returns: (transfer none) (nullable): The pointer to native credentials or
+ *     %NULL if there is no #GCredentials support for the OS or if @native_type
+ *     isn't supported by the OS. Do not free the returned data, it is owned
+ *     by @credentials.
  *
  * Since: 2.26
  */
@@ -507,7 +507,7 @@ g_credentials_set_native (GCredentials     *credentials,
  * OS or if the native credentials type does not contain information
  * about the UNIX user.
  *
- * Returns: The UNIX user identifier or -1 if @error is set.
+ * Returns: The UNIX user identifier or `-1` if @error is set.
  *
  * Since: 2.26
  */
@@ -572,7 +572,7 @@ g_credentials_get_unix_user (GCredentials    *credentials,
  * about the UNIX process ID (for example this is the case for
  * %G_CREDENTIALS_TYPE_APPLE_XUCRED).
  *
- * Returns: The UNIX process ID, or -1 if @error is set.
+ * Returns: The UNIX process ID, or `-1` if @error is set.
  *
  * Since: 2.36
  */
diff --git a/gio/gdbusaddress.c b/gio/gdbusaddress.c
index 3dd3cc84b..26d52ea36 100644
--- a/gio/gdbusaddress.c
+++ b/gio/gdbusaddress.c
@@ -903,11 +903,14 @@ g_dbus_address_get_stream (const gchar         *address,
 /**
  * g_dbus_address_get_stream_finish:
  * @res: A #GAsyncResult obtained from the GAsyncReadyCallback passed to g_dbus_address_get_stream().
- * @out_guid: (optional) (out): %NULL or return location to store the GUID extracted from @address, if any.
+ * @out_guid: (optional) (out) (nullable): %NULL or return location to store the GUID extracted from @address, if any.
  * @error: Return location for error or %NULL.
  *
  * Finishes an operation started with g_dbus_address_get_stream().
  *
+ * A server is not required to set a GUID, so @out_guid may be set to %NULL
+ * even on success.
+ *
  * Returns: (transfer full): A #GIOStream or %NULL if @error is set.
  *
  * Since: 2.26
@@ -940,7 +943,7 @@ g_dbus_address_get_stream_finish (GAsyncResult        *res,
 /**
  * g_dbus_address_get_stream_sync:
  * @address: A valid D-Bus address.
- * @out_guid: (optional) (out): %NULL or return location to store the GUID extracted from @address, if any.
+ * @out_guid: (optional) (out) (nullable): %NULL or return location to store the GUID extracted from @address, if any.
  * @cancellable: (nullable): A #GCancellable or %NULL.
  * @error: Return location for error or %NULL.
  *
@@ -949,6 +952,9 @@ g_dbus_address_get_stream_finish (GAsyncResult        *res,
  * of the D-Bus authentication conversation. @address must be in the
  * [D-Bus address format](https://dbus.freedesktop.org/doc/dbus-specification.html#addresses).
  *
+ * A server is not required to set a GUID, so @out_guid may be set to %NULL
+ * even on success.
+ *
  * This is a synchronous failable function. See
  * g_dbus_address_get_stream() for the asynchronous version.
  *
diff --git a/gio/gdbusconnection.c b/gio/gdbusconnection.c
index 65939a4d2..5c5ef1718 100644
--- a/gio/gdbusconnection.c
+++ b/gio/gdbusconnection.c
@@ -885,7 +885,7 @@ g_dbus_connection_class_init (GDBusConnectionClass *klass)
    *
    * If you are constructing a #GDBusConnection and pass
    * %G_DBUS_CONNECTION_FLAGS_AUTHENTICATION_SERVER in the
-   * #GDBusConnection:flags property then you MUST also set this
+   * #GDBusConnection:flags property then you **must** also set this
    * property to a valid guid.
    *
    * If you are constructing a #GDBusConnection and pass
@@ -1102,7 +1102,7 @@ g_dbus_connection_init (GDBusConnection *connection)
  * stream from a worker thread, so it is not safe to interact with
  * the stream directly.
  *
- * Returns: (transfer none): the stream used for IO
+ * Returns: (transfer none) (not nullable): the stream used for IO
  *
  * Since: 2.26
  */
@@ -2732,7 +2732,7 @@ g_dbus_connection_new (GIOStream            *stream,
  *
  * Finishes an operation started with g_dbus_connection_new().
  *
- * Returns: a #GDBusConnection or %NULL if @error is set. Free
+ * Returns: (transfer full): a #GDBusConnection or %NULL if @error is set. Free
  *     with g_object_unref().
  *
  * Since: 2.26
@@ -2784,7 +2784,8 @@ g_dbus_connection_new_finish (GAsyncResult  *res,
  * This is a synchronous failable constructor. See
  * g_dbus_connection_new() for the asynchronous version.
  *
- * Returns: a #GDBusConnection or %NULL if @error is set. Free with g_object_unref().
+ * Returns: (transfer full): a #GDBusConnection or %NULL if @error is set.
+ *     Free with g_object_unref().
  *
  * Since: 2.26
  */
@@ -2875,8 +2876,8 @@ g_dbus_connection_new_for_address (const gchar          *address,
  *
  * Finishes an operation started with g_dbus_connection_new_for_address().
  *
- * Returns: a #GDBusConnection or %NULL if @error is set. Free with
- *     g_object_unref().
+ * Returns: (transfer full): a #GDBusConnection or %NULL if @error is set.
+ *     Free with g_object_unref().
  *
  * Since: 2.26
  */
@@ -2927,8 +2928,8 @@ g_dbus_connection_new_for_address_finish (GAsyncResult  *res,
  * If @observer is not %NULL it may be used to control the
  * authentication process.
  *
- * Returns: a #GDBusConnection or %NULL if @error is set. Free with
- *     g_object_unref().
+ * Returns: (transfer full): a #GDBusConnection or %NULL if @error is set.
+ *     Free with g_object_unref().
  *
  * Since: 2.26
  */
@@ -3017,7 +3018,7 @@ g_dbus_connection_get_exit_on_close (GDBusConnection *connection)
  * The GUID of the peer performing the role of server when
  * authenticating. See #GDBusConnection:guid for more details.
  *
- * Returns: The GUID. Do not free this string, it is owned by
+ * Returns: (not nullable): The GUID. Do not free this string, it is owned by
  *     @connection.
  *
  * Since: 2.26
@@ -5510,7 +5511,7 @@ register_with_closures_on_set_property (GDBusConnection *connection,
  * Version of g_dbus_connection_register_object() using closures instead of a
  * #GDBusInterfaceVTable for easier binding in other languages.
  *
- * Returns: 0 if @error is set, otherwise a registration id (never 0)
+ * Returns: 0 if @error is set, otherwise a registration ID (never 0)
  * that can be used with g_dbus_connection_unregister_object() .
  *
  * Since: 2.46
@@ -6139,8 +6140,8 @@ g_dbus_connection_call (GDBusConnection     *connection,
  *
  * Finishes an operation started with g_dbus_connection_call().
  *
- * Returns: %NULL if @error is set. Otherwise a #GVariant tuple with
- *     return values. Free with g_variant_unref().
+ * Returns: (transfer full): %NULL if @error is set. Otherwise a non-floating
+ *     #GVariant tuple with return values. Free with g_variant_unref().
  *
  * Since: 2.26
  */
@@ -6206,8 +6207,8 @@ g_dbus_connection_call_finish (GDBusConnection  *connection,
  * g_dbus_connection_call() for the asynchronous version of
  * this method.
  *
- * Returns: %NULL if @error is set. Otherwise a #GVariant tuple with
- *     return values. Free with g_variant_unref().
+ * Returns: (transfer full): %NULL if @error is set. Otherwise a non-floating
+ *     #GVariant tuple with return values. Free with g_variant_unref().
  *
  * Since: 2.26
  */
@@ -6309,8 +6310,8 @@ g_dbus_connection_call_with_unix_fd_list (GDBusConnection     *connection,
  * access file descriptors if they are referenced in this way by a
  * value of type %G_VARIANT_TYPE_HANDLE in the body of the message.
  *
- * Returns: %NULL if @error is set. Otherwise a #GVariant tuple with
- *     return values. Free with g_variant_unref().
+ * Returns: (transfer full): %NULL if @error is set. Otherwise a non-floating
+ *     #GVariant tuple with return values. Free with g_variant_unref().
  *
  * Since: 2.30
  */
@@ -6348,8 +6349,8 @@ g_dbus_connection_call_with_unix_fd_list_finish (GDBusConnection  *connection,
  *
  * This method is only available on UNIX.
  *
- * Returns: %NULL if @error is set. Otherwise a #GVariant tuple with
- *     return values. Free with g_variant_unref().
+ * Returns: (transfer full): %NULL if @error is set. Otherwise a non-floating
+ *     #GVariant tuple with return values. Free with g_variant_unref().
  *
  * Since: 2.30
  */
@@ -6859,8 +6860,8 @@ subtree_message_func (GDBusConnection *connection,
  * See this [server][gdbus-subtree-server] for an example of how to use
  * this method.
  *
- * Returns: 0 if @error is set, otherwise a subtree registration id (never 0)
- * that can be used with g_dbus_connection_unregister_subtree() .
+ * Returns: 0 if @error is set, otherwise a subtree registration ID (never 0)
+ * that can be used with g_dbus_connection_unregister_subtree()
  *
  * Since: 2.26
  */
diff --git a/gio/gdbusconnection.h b/gio/gdbusconnection.h
index 05ef384e4..4bd3e9a4b 100644
--- a/gio/gdbusconnection.h
+++ b/gio/gdbusconnection.h
@@ -432,11 +432,11 @@ gboolean         g_dbus_connection_unregister_object          (GDBusConnection
  * specified (ie: to verify that the object path is valid).
  *
  * Hierarchies are not supported; the items that you return should not
- * contain the '/' character.
+ * contain the `/` character.
  *
  * The return value will be freed with g_strfreev().
  *
- * Returns: A newly allocated array of strings for node names that are children of @object_path.
+ * Returns: (array zero-terminated=1) (transfer full): A newly allocated array of strings for node names that are children of @object_path.
  *
  * Since: 2.26
  */
@@ -472,7 +472,7 @@ typedef gchar** (*GDBusSubtreeEnumerateFunc) (GDBusConnection       *connection,
  * remote introspector in the empty array case, but not in the %NULL
  * case.
  *
- * Returns: A %NULL-terminated array of pointers to #GDBusInterfaceInfo, or %NULL.
+ * Returns: (array zero-terminated=1) (nullable) (transfer full): A %NULL-terminated array of pointers to #GDBusInterfaceInfo, or %NULL.
  *
  * Since: 2.26
  */
@@ -497,7 +497,7 @@ typedef GDBusInterfaceInfo ** (*GDBusSubtreeIntrospectFunc) (GDBusConnection
  * Subtrees are flat.  @node, if non-%NULL, is always exactly one
  * segment of the object path (ie: it never contains a slash).
  *
- * Returns: A #GDBusInterfaceVTable or %NULL if you don't want to handle the methods.
+ * Returns: (nullable): A #GDBusInterfaceVTable or %NULL if you don't want to handle the methods.
  *
  * Since: 2.26
  */
diff --git a/gio/gdbuserror.c b/gio/gdbuserror.c
index 4ad97bb6e..b3ea28f5a 100644
--- a/gio/gdbuserror.c
+++ b/gio/gdbuserror.c
@@ -512,8 +512,8 @@ g_dbus_error_is_remote_error (const GError *error)
  * (e.g. g_dbus_connection_call_finish()) unless
  * g_dbus_error_strip_remote_error() has been used on @error.
  *
- * Returns: an allocated string or %NULL if the D-Bus error name
- *     could not be found. Free with g_free().
+ * Returns: (nullable) (transfer full): an allocated string or %NULL if the
+ *     D-Bus error name could not be found. Free with g_free().
  *
  * Since: 2.26
  */
@@ -600,7 +600,7 @@ g_dbus_error_get_remote_error (const GError *error)
  * #GError instances for applications. Regular applications should not use
  * it.
  *
- * Returns: An allocated #GError. Free with g_error_free().
+ * Returns: (transfer full): An allocated #GError. Free with g_error_free().
  *
  * Since: 2.26
  */
@@ -810,7 +810,8 @@ g_dbus_error_strip_remote_error (GError *error)
  * This function is typically only used in object mappings to put a
  * #GError on the wire. Regular applications should not use it.
  *
- * Returns: A D-Bus error name (never %NULL). Free with g_free().
+ * Returns: (transfer full) (not nullable): A D-Bus error name (never %NULL).
+ *     Free with g_free().
  *
  * Since: 2.26
  */