Gio.Settings.schema

Settings.schema: String (Read / Write / Construct Only)
  

The name of the schema that describes the types of keys for this Gio.Settings object.

The type of this property is *not* Gio.SettingsSchema. Gio.SettingsSchema has only existed since version 2.32 and unfortunately this name was used in previous versions to refer to the schema ID rather than the schema itself. Take care to use the 'settings-schema' property if you wish to pass in a Gio.SettingsSchema.