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.
I've been working away at this for quite awhile now and after many revisions I'm happy to release version 1.0 of the new Source Multi-Tool. Source Multi-Tool is meant as a replacement to Valves Source SDK Launcher, as of right now all it does is launch hammer or authoring tools as well as being able to configure a custom config for you but in the future it is planned to do much more. What is custom config? Custom config is a feature I setup so that you can quickly setup a custom hammer config. Want to map for garrysmod and have CSS, DoD and HL2 content all load up without having to unpack crap and eat your disk space? This lets you do that!
Be sure to join the Steam Group page if you want to know right away when a new version is released and any news on it: link
If you find a bug please post it on the group hub page so that they are all in one place.