Firstly, DAH! has an extensive roster of characters ranging through 3 different decades (50’s, 60’s, and 70’s)
and in porting them it gives GMOD a good variety for period works. Also some killer Aliens and wicked Technology, and who doesn’t love dancing cows?
Psychonauts, what could be said about Psychonauts’ characters besides they’re incredibly inventive and awesome.
The Suffering has a wonderful bestiary, some of the most horrific looking things, and has a lot of lore tied around the series which adds to that.
Psi-Ops has a great selection of pseudo-military forces and scientists.
First off, please update your post with what you want ported, preferably with screenshots, and keep in mind that not everything you ask for will be done. I second the destroy all humans request, however, be aware that most of the ragdolls would look rather ugly in source because of their low res textures. The vehicles might fare better, though. The suffering already looks bad in-game, but the monsters might be a nice addition to gmod. Can’t say much for your other two requests, although I do see some potential in psi-ops.