I recently got Garry's Mod (or more commenly known as Gmod) for $2.50 on Steam. So, here's my review

Graphics 8/10:
The graphics truly depend on how high the settings are at. Mine are on low, and yet the graphics are good.The models are from other games, so depending on the game, that is how detailed they are.

Sounds 8/10:
The sounds are from other games, but they are really great though. There is no music though. Just a bunch of SFX, which are great.

Gameplay 10/10:
The game is amazing. In Gmod, you are given hundreds (even thousands, depending on how many Valve games you have) of items to play around with. You can create anything, from a hover chair with a gatling gun, to a tractor with a catapult that lobs explosive barrels. The game is multiplayer too, so a player can make a zombie survival map with their friends and see how long they last. Gmod will take objects (NPC's, models and such) from other Valve Source games (such as Half Life 2 and Counter Strike) which add even more possibilities for creations.

Overall: 9/10

Gmod is full of endless opportunities. It supports mods too, which can add all sorts of crazy things (even Steve from Minecraft)! There is an in game browser, where you can download maps, props, models, and weapons. This game is a great game for anyone!