Gtk3 is a newer version and it works a lot better then Gtk2 in many ways look, feel, speed and usage. There could be some old platforms that may only run Gtk2. But if you can use the newest version then do so...
There are many others using gnocl, QT. For portability QT looks like the winner but is written in C++.

I think the funny thing about all the GUI stuff I just read is. A kinda new feature in Gtk3 that makes the GUI more uniform cross platform and easier to design is the use of CSS in the GUI.
You can also write the GUI in HTML a lot faster then using some of the above programs. I see HTML as being a true URGUI (Universal Remote Graphical User Interface) in this time and age, but you don't need portability like you said. ッ