Sorry about that, we're trying to fix up our Travis Continuous Integration 100% so that it will actually tell us when a pull request on GitHub breaks the engine:https://github.com/enigma-dev/enigma-dev/pull/1077
I've actually made a number of changes to the GTK system that should improve how it finds the headers on Linux. You should probably take a look at that which is going to be merged tomorrow.
What platform are you trying to use gtk from? This should fix Linux, but for Windows, you'll want to download the GTK headers manually and then stick them in:
Edit: Oh sorry, I didn't know you were asking about the GUI extension, TheExDeus wrote that and it may only work if you change the graphics system to OpenGL 3.3, I'm not exactly sure as I never tested it myself.