In my (biased) opinion and experience, you're better off learning GTK by command-line compilation and your favorite editor (gedit, kate, vi, emacs, whatever). This way, you can learn at your own pace rather than trying to grapple with a big complicated IDE that really isn't beginner-friendly. Nonetheless, be aware of devhelp (GTK's development documentation program) and try building a couple GUIs with glade3 and using them in your C programs.
This might not be the answer you want, but I feel that C/C++ GUI IDEs tend to suck, at least for beginners.
Anjuta can do C/GTK+, but I personally wasn't very impressed with it. It asks you what plugin you want to open .glade files with, new projects are built with autoconf (resulting in a mess of over 70 files for a simple "Hello world") and localized with gettext by default (resulting in a bunch of boilerplate code in main.c), and it pops dialogs like this when you invoke weird edge cases such as double clicking a button you just created:
My impression of Anjuta from the perspective of a beginner was, as you can tell, highly negative. It shows a whole lot of advanced options, but doesn't let you do basic tasks without a lot of hassle. Anjuta is not alone. In general, I don't believe I've ever found a (mature) C/C++ IDE for any GUI toolkit that was easy for a beginner like me.
