GMod is pretty difficult to describe lol.... Here's the Urban Dictionary definition xD
"A modification of the game Half-Life 2. It allows you to experiment with the game's physics engine by posing ragdolls or building vehicles out of random objects. It also allows you to paint any ragdoll or object and you can change the physical properties of the object or ragdoll."
"Garry's Mod (also known as GMod in short) is a Source Engine modification which gives players a number of tools and the ability to spawn any object found in the Source Engine and do as they please, such as making contraptions or posing ragdolls."
"A addictive build server, with alot of crap. You can build a human car, a boat, a melon gun, facepoze and make movies kinda."