What game on steam do you need to get to get garrys mod

Any Source game.

This includes:

Half-Life 2 (You pretty much get stuff from this in any Source game)
Half-Life 2: Episode 1

Half-Life 2: Episode 2
Team Fortress 2
Portal
Counter-Strike: Source
Day of Defeat: Source

Get any 2 or more of these and you should be set to go, but I really honestly suggest getting all of them, its only about $50 (if you get the $30 Orange Box, which contains all Half-Life 2 games, Portal, and Team Fortress 2, then get CS:S for $20)

Alternatively, Team Fortress 2 is now Free-to-Play and downloading that will allow you to play Gary's Mod, or any other Source mod!