Please try this:
$ (cd /usr/include/gtk-2.0; gtags) # make tag in the library project
$ gtags # make tag in your project
$ GTAGSLIBPATH=/usr/include/gtk-2.0; export GTAGSLIBPATH
$ global -x xxx
If 'xxx' exists in your project, global show it. Otherwise, if 'xxx' exists in the library projects, global show it.
To show both of them, please try this:
$ GTAGSTHROUGH=; export GTAGSTHROUGH
If you have many libraries then please specify them like PATH environment variable.
$ GTAGSLIBPATH=/usr/src/lib:/usr/include/gtk-2.0; export GTAGSLIBPATH
Good luck!