Garry's Mod is a sandbox game. There aren't any predefined aims or goals like in regular games.
Players are given tools and are left to entertain themselves.
It could be used to create comic strips, by posing ragdolls and taking screenshots.
It could be used to create and race cars, by welding objects together and adding wheels and thrusters.
It could be used to create a house by placing furniture and walls carefully around the map and inviting your friends to explore.