It will probably only list modules that you've installed via the standard perl Makefile.PL; make; make test; make install (or the Build.PL equivalent) dance. This includes packages installed via the CPAN client. But if you've installed them some other way (e.g. your operating system's package manager) then that might bypass perllocal.pod.

I personally install absobloodylutely everything using cpanminus (or actually, my fork of cpanminus, called cpantimes), and my perllocal.pod seems pretty accurate.

I don't have Gtk2 on my current working version of Perl, but looking at perllocal.pod for my older installation, I see:

   Tue Jan 15 14:22:03 2013: "Module" Gtk2
       ·   "installed into: /home/tai/.perlbrew/libs/perl-5.16.2@defau
+lt/lib/perl5"

       ·   "LINKTYPE: dynamic"

       ·   "VERSION: 1.246"

       ·   "EXE_FILES: "
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