Instead of:

warning: ‘g_variant_get_gtype’ is deprecated (declared at ../../gobject/glib-types.h:242): Use '((GType) ((21) << (2)))' instead [-Wdeprecated-declarations]

show:

warning: ‘g_variant_get_gtype’ is deprecated (declared at ../../gobject/glib-types.h:242): Use ''G_VARIANT_GET_TYPE'' instead [-Wdeprecated-declarations]

Also, document the macro-expansion problem in the
G_GNUC_DEPRECATED_FOR docs