GNOME doesn't have it's own theme, it uses whatever GTK uses, i.e. Adwaita by default. Gnome Shell, which is one of the few parts that does have it's own theming, doesn't use GTK at all.

The reason why theming was unstable is, that it was being reimplemented. That's a thing you cannot do overnight, it takes some time. So if you want a theme that is not adwaita, you have to do exactly the same thing that the adwaita people did: update it for each release. Yes, that takes time too.

Doing things in explicitly marked unstable internals of any framework will get you the authors protecting themselves against you. In Windows, the theme authors got signed themes, in MacOS they got SIP. Yes, it is not a insurmountable problem to avoid both, that's not their purpose, but when something breaks, it is obvious who's fault it is.

GIMP is still Gtk 2 app, so it wasn't concerned with broken theming at all. They have their own work to do with the switch to GEGL.