Garry's Mod is a sandbox type game that allows users to manipulate props and build creations, with no actual goal or object to achieve. Most of the props used in the game are from the source game "Half-Life 2" and other games running on Valve's "Source Engine". The game consists of many user-made game modes including Role-playing, adventure, action, etc. Because of this the game does not have a specific genre.
 Game Play
Users start off with tools used to manipulate world objects and props. Tools such as, the "Physics Gun" which allows the rotation and moving of objects. Other tools such as the "Tool Gun" can be used to add other effects to the props such as colors, weight and size. Props can be inserted into the game by holding down the "Q" key and selecting an object from the in-game menu. The game is based off of the title "Half-Life 2", so most of the original game controls apply in Garry's Mod. Users also have the ability to play online on servers created by other users. Most of the servers have different "Game Modes" installed, or mods that change the way the server works. Users download the files required before joining the game.
The game allows users to insert their own "Mods" into the game through the game's program files. After doing this, when you start a new single player game the new objects can be found on the "Spawn Menu".
Online play can be accessed at the menu, where users can play different "game modes" and mods installed on the server.
If you buy a "Source game" from the Steam store, most (such as "Half-Life 2: Episode 2" or "Portal") will automatically add it's game files to Garry's Mod. Allowing users to play with the objects from that game.