GTK+ installation

You should use : apt-get install libgtk2.0-dev

You don't need the git clone step. This would be required only if you wanted to rebuild GTK+.

What you want is the development packages of your Linux distro. Search in your packages for something like libgtk*-dev, and install it. This will install the headers you need and all the dependencies required. Furthermore, pkg-config won't require any hint to find the .pc files that describe how to build and link applications against those libraries, so no need for configuring PKG_CONFIG_PATH.