I dont have Half-Life 2 but when i check it out on steam it says that:
This game requires that you own a Source engine game such as Counter-Strike: Source, Half-Life 2, or Day of Defeat: Source. Click here for a full list of games that meet this requirement. Please note that Portal: First Slice and the bundled Half-Life 2: Deathmatch do not fulfill this requirement.
And when you click the “click here” button it shows a list of games that would work which i currently own Left4Dead 1 & 2, as well as Team Fortress 2. Can i still play garys mod?