If that could help - my searching program can't find gtk.h file.
If you don't have gtk.h on your system, it can not be found (obviously).
Install the package which includes the gtk.h you need (either gtk2 or gtk3).
It's up to you to find out which package is the correct one.
This website/forum is not dedicated to find out such stuff.
We can help to find the correct place in C::B to put the search path in.
For libraries you should always use pkg-config to set the needed variables or you will fail sooner or later, if a package is updated and some stuff changes.
Searching the forum for pkg-config should (hopefully) guide you to an explananation on how to use it correctly (at least in general, not for each and any special lib/toolkit of course).