@wth I have worked with GTK3 myself, and once I got used to its quirks, actually found it quite nice to work with. I was writing my code in Python too, which added some extra challenge, but the GObject introspection took a lot of the pain out of interoperating with what's basically a C library.
However, I'm aware that GTK has a bit of a reputation. The look and feel is great on Linux desktops that use it natively, but I do remember it looking pretty ugly cross-platform.