Yes, that's great! Source is always a welcome way to view intent of design for me! I'll do a deep dive this evening.
The key takeaway here is GTK's developer support stuff is built with GTK.
And from a philosophical standpoint: no matter how good the HUD becomes, you will eventually out grow it over the lifetime of a non-trivial game. "Batteries include" tool chains are really great getting started, but it doesn't show the dev that it's just as "easy" to extend the tool chain and tune it such that it "grows" with the application being built.
I'd encourage you to make your own HUD that's customized/optimized for the work you are trying to do (I definitely looks like you have a grasp of advanced Ruby concepts and I think you'll make some really amazing things).