How long would it take to learn and use GTK+?

Off-topic: It will be quicker to use Python with GTK+.

Completely agree with Nguyen. Python will be easier. You sound like a beginner in programming, learning basic python will take around 5-6 days at max(invest your time in some awesome MOOC like http://www.edx.org or http://www.Udacity.com) PyGtk+(http://www.pygtk.org/pygtk2tutorial/index.html) . Python(http://www.python.org/doc/) As you want to contribute in Gnome, finding any project with Python + GTK  combination will be easier.All project on Gnome site(https://wiki.gnome.org/Apps) and there source code will be available either on http://github.com or https://git.gnome.org/browse/  Hail Open Source..!!

A2A! Its great that you want to learn GTK+. There are a few things you should keep in mind: 1. In C, creating any kind of software is more challenging as compared to writing it in any OOPS language. 2. In C, you can also do Object Oriented Programming though C doesn't have constructs which could allow you to do OOP easily. 3. It will be good to learn some GUI concepts before starting GTK+. 4. You must be good with handling pointers. As said. There are many tutorials available. Three I found best are: https://developer.gnome.org/ http://www.zetcode.com http://scentric.net/tutorial Go through each of these tutorials and you will have a good knowledge of GTK+. You can also found GTK+ Reference Manual on https://developer.gnome.org/ . Reference Manual or API Reference is very helpful, as you will not always remember each and every function. This book is also very good : Apress Foundations of GTK+ Development You may have a problem that most of the tutorials are of GTK+ 2.0 while latest of GTK+ 3.x. Well, still GTK+ 2.0 is being used and I think has been the most popular version of GTK+ so far. Go, with anyone. The basics of both the versions are same. Mostly tutorials of GTK+ 2.0 will be compatible with GTK+ 3.0. I think no time is enough to learn GTK+ completely. It is a very big framework. All you can do is, learn while you develop. This is the approach I have followed and has been very great. At last, try to develop something which uses GTK+ heavily. Also, try to learn how to do OOP with C it will be very helpful in the long run.