There are a lot of dependencies in order for you to install gtk. I have installed mine but I've got lot's of package lacking on my Distro, atk, pango, cairo to name a few. Be sure to have them first and make sure to visit the site John A posted. :)
All of you have failed miserably to answer this guys question. It's no bloody wonder more people don't understand programming. I'm in second year programming in uni and I need Gtk# 2.12.9 or greater to install MonoDevelop, and you're telling me Gtk requires other "dependencies"? this is complete nonsense in my opinion but I need this answer because I can't understand the lingo tossed around in "help" forums.