Do you need half life 2 to play garrys mod

No you do not. You just download Garry's mod and it comes with the Half life 2 models. Your Welcome. Add me on steam. Killermuffinofdoom

Incorrect - As stated on the page of Garry's mod on steam, you require a source engine game. a full list of valid games are available here: http://store.steampowered.com/search/?category2=16