The functions are private, and an implementation detail of the macros.
https://bugzilla.gnome.org/show_bug.cgi?id=700035
g_type_get_type_registration_serial
G_DEFINE_TYPE
+G_DEFINE_TYPE_WITH_PRIVATE
G_DEFINE_TYPE_WITH_CODE
G_DEFINE_ABSTRACT_TYPE
+G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE
G_DEFINE_ABSTRACT_TYPE_WITH_CODE
+G_ADD_PRIVATE
+G_PRIVATE_OFFSET
+G_PRIVATE_FIELD
+G_PRIVATE_FIELD_P
G_DEFINE_INTERFACE
G_DEFINE_INTERFACE_WITH_CODE
G_IMPLEMENT_INTERFACE
g_type_check_is_value_type
g_type_check_value
g_type_check_value_holds
+g_type_class_adjust_private_offset
+g_type_add_instance_private
g_type_instance_get_private
g_type_class_get_instance_private_offset
g_type_class_get_private