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.
Garry's mod is a Sandbox mod, it's very difficult to give a "rating" to a game where you just build and do what you want.
The criteria to judge it are, in my opinion the following:
-Tools at your disposal
-Amount of possibilities
-Flexibility of the engine
-Stability of the engine
-How often is the game updated
And by those criteria, I can say that Garry's Mod deserves a 10, with countless tools, that can even be downloaded, same as for maps and models, and even LUA scripts that was implemented into the Source Engine (one of the best ideas ever), Garry's Mod is not only a great Sandbox Mod, it is also one of the best Sandbox GAMES, right next to Minecraft.
In addition to that the Mod has been constantly updated, adding new tools, new features and fixing problems very often.
This is one of the most played mods for the Source Engine (if not THE most played mod)
Well deserved success.
This game USED to be fantastic; one of the best games ever made. But then the Garry's Mod 13 update utterly ruined every aspect of the game! Saves are broken, loading times are way too long, the Workshop is blatantly worse than its predecessors (garrysmod.org & Toybox,) the UI is downright ugly, its so badly optimized even some HIGH-END PCs can have trouble running it, for a while there was an exploit that allowed your computer to become infected with a virus via Gmod servers, even BASIC features like the Inflator Tool don't work properly anymore. And don't expect the game to EVER get decent bugfixes again; it has been abandoned by its developers entirely.
The games functionality isn't the only thing that has become awful over the years; the Gmod community itself has become one of the worst communities I've seen in gaming. You will rarely find a friendly/smart person in it anymore.
DO NOT BUY THIS GAME. It is only a shell of what it used to be.
Also garry supports paid mods. So there's that.
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.