You will also need to add "extension=php_gtk2.so" to your /etc/php5/cli/php.ini after installing the package.
Michael: you should definitely add a link for that package to the download section of the site; maybe even a x86 version or at least a source deb that can be used to compile it.
Just this week I installed PTS on a ubuntu pc to test the pc, and wanted to leave it with a shiny gui that I can show my friend, and gave up after all the confusing and outdated instructions on getting php5-gtk running on ubuntu.
I think it's mentioned in the HTML documentation. I wish I had my build scripts still that I used to easily come up with that package, but I don't. I sure wish though php5-gtk would be more widely packaged...