GMod is a Sandbox Modification. Using GMod you can pose ragdolls. This means can position people from HL2/CS/DoD and change their faces. A big comic book creating community has popped up around this feature. Using GMod you can build. Take props from HL2/CS/DoD and weld them together to make walls. Axis a wheel to it to create a working car. Rope some cans to the back. A big contraption creating community has popped up around GMod - creating such things as Rube Goldberg devices, catapults, cannons, bridges etc.. Using GMod you can make your own weapons. That's right, make your own weapons. Using Lua you can create a custom weapon to do just about anything. If you're new you can change simple things like rate of fire, models, sounds. If you're awesome the world is your oyster, mass changing guns, melon cannons, jetpacks. GMod gives you the freedom to do any of these things in single player or online with your friends.
While there are insurmountable shortcomings to this mod (or the full release as GMod 10), it has without a doubt become one of the cornerstones to the Source engine as a modding tool, or a platform for LUA scriptors. I cannot begin to describe the literal plethora of fun gamemodes I've tested over the possible 7 or so years since I started playing this mod, back when it had basic prism blocks spawn from emitter constructs.
While it is beyond perfect, it is one of the best intros for mappers, modders and scriptors alike.