That GTK_GRID bit is a macro that evaluates at runtime and will chuck out runtime errors if the widget doesn't match. It supports inheritance. If you have for example a gtk_combo_box_text, you can use gtk_combo_box_set_active(GTK_COMBO_BOX(my_combo_box_text), 0) and it will select the first entry in the gtk_combo_box_text because gtk_combo_box_text inherits from gtk_combo_box like any good object oriented system.
It looks like this when you program it:
That GTK_GRID bit is a macro that evaluates at runtime and will chuck out runtime errors if the widget doesn't match. It supports inheritance. If you have for example a gtk_combo_box_text, you can use gtk_combo_box_set_active(GTK_COMBO_BOX(my_combo_box_text), 0) and it will select the first entry in the gtk_combo_box_text because gtk_combo_box_text inherits from gtk_combo_box like any good object oriented system.