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!
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i recommend the vavle pack valve is the creators of steam if u have good gameing PC tho and garrys mod but u must have some of the valve games to play garrys mod great game!
no because garrys mod requires that you have a game that uses the source engine
All steam games update themselves automatically.
You can buy it at the Steam Store. Check the related link.
10 is better, $10 Steam