GD for some reason is not part of ActiveState PPM

That's because GD doesn't build under ActiveState's automated procedure. You can, however, install via ppm from randyk's ppm repository. Just run:

ppm install http://theoryx5.uwinnipeg.ca/ppms/GD.ppd
[download]

See http://theoryx5.uwinnipeg.ca/ppms/ for a complete list of the modules available at randyk's rep, and add it to ppm's list of repositories. (Another useful rep can be found at http://www.bribes.org/perl/ppmdir.html.)

Cheers,
Rob