It's been a hot minute, since I've been to the GTK side of town, but I'm aware of Geany and GNOME Builder (I believe, the latter is focused on developing GNOME apps, but not sure to what degree).
Well, and I guess, there's also gVim and Emacs, if you're into very keyboard-driven editors...
Personally, I like vim. I do miss the mini map to speed up navigating through code, but I don't think I've ever seen a random crash or have it inadvertently fill up my home dir because I've had it open for too long.