That's really weird to hear. If you build an app with GTK3 today, it's still going to work for 10 years. It's compiled into the application, it's not a runtime API. Even if you upgrade GNOME to some futuristic 10 year version, the GTK3 compiled app will just continue to work fine.
The parent thread is comparing a runtime API (Win32) with a compile-time library (GTK).