Not sure what you’re referring to.

Gtk 1.2 did not really have great Win32 support, was basically a one man show primarily to get gimp to run on Windows. The Win32 backend took off after gtk2, there was more commercial push from Ximian to make it a cross platform toolkit via Gtk# as well. And it wasn’t dropped in 3 or 4, though it definitely has lost development steam as commercial interest in it and cross platform desktop GUI toolkits in general has flagged and consolidated.