Hi! Welcome to the Chan!
I've never used GMod before in my life, however, I am very much a 2D animation enthusiast and I can tell you straight off the bat that you'll need to perfect (well, okay... practice) a basic walk cycle before anything else. I've included an image with a few different kinds of walk cycles for your viewing pleasure.
Don't try be a hero on your first go, just take things nice and steady- stop motion puts you in for a long haul, and it's gonna be tedious as hell. Oh and remember, run cycles have a moment in which both feet are in the air, walk cycles don't- that's why they look like completely different motions.
Watch a lot of stop-motion stuff! Raikujo, RubberFruit (ssshhhhh, now, guys) and Kelthuzadmadness are all GMod stop-motion artists I can name off the top of my head. Don't be afraid to take a gander at a few non-stop-motion types, as well. I hear MaxOfS2D whips up a mean short.
Lastly, this isn't something I've learned from using any animation program, but rather from back in high school when I had to compose a piece of music on the computer; DO NOT repetitively view recently completed chunks of animation (or any media, really). Soon each chunk will stick out individually like a sore thumb and they'll start to feel tedious for you to watch (even if your mate thinks it's fantastic). When you've completed a chunk, watch it once, maybe twice if you're unsure, and then either stow it or edit the more glaring flaws.
Err... that's all I can think of. Hope I haven't bored you out of your skull. Enjoy your stay on the chan!