“The Type or Namespace ‘GTK’ could not be found” error – MonoDevelop on Linux

MonoDevelop 3.0.6
GTK# 2.12
openSuse 13.1

When you press F5 to build and get this error in MonoDevelop, just go to:

Project -> Your Project name Options -> Build -> General

You probably see:

Target Framework: Mono /.Net 3.5

Change it to:

Target Framework: Mono /.Net 4.0

Do all what MonoDevelop wants from you. Now, your project has no errors during building.