A guest post from GeekTeen John Ceceri: Garry’s Mod (GMod) is a program made by Garry Newman that allows you to manipulate the Half-Life and Source engine games. If you’ve ever searched ...

That's cleared up, let's address the 'how'. You'll need Garry's Mod 13, the current beta that Garry's been developing for the last year. If you have GMod on Steam you'll now have access to the beta.