That's funny. Because I remember one of the changes from gtk-1.2 to gtk-2.0 (or maybe 2 to 3?) was to drop Win32 support. Which I at the time didn't like. We go full circle.
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.
You're right that gdk2 ramped up Win32 support, I remember that now.