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.