Garry's Mod or simply GMod is a source-engine based modification that now works as a standalone game. With no plot or exact gameplay at all, anyone can create his own game mode to play for fun. You can find anything you're into. Trouble In Terrorist Town, Prop Hunt, Hunger Games, Franchise-based modes such as Star Wars or Lord of the Rings - anything.

Basically, GMod stands for God Mode in video games, because you can play with any asset of the source engine. Try out physics, characters, maps, weaponry - anything. Based on that, Garry's Mod gives players quite a variety of mods to spend their time. It doesn't sound so earnestly. You will probably find something that will provide you with quite a good amount of fun. Servers are specifically sorted in game modes so you'll never play a different mode while playing exact one what you wanted. At the end of the day, this game is about receiving as much fun from what is happening around you as possible.