I generally feel that the docs for various language bindings like pygobject and gtk-rs are much better then the docs for C.
I really want to complain that the current status of GTK is just “black art” with many parts lacking documentation. The many parts of GTK like gtkbuilder, gresources, gobject are completely confusing for newcomers, yet these are essential for GTK applications to work. The gtkbuilder’s xml format is even undocumented (not even those xml tags and attributes already supported by GTK, let alone create out-of-tree subparsers) , and glade does not even recognize the official demo’s xml definition.
Update: glade won’t support GTK4. It was intended in glade-users-list, but was dropped several months later here. The glade maintainer has a GUADEC presentation about the new Cambalache UI maker and also blogged Merengue: Cambalache’s workspace. Also see Plan about GTK4 support of Glade? - #2 by sophieherold