Do I requireHalf-Life 2 and Counter strike: source for the GMOD version on steam? (the one you buy) Or will Half-life 2 suffice? Thanks in advance.
No, you only need hl2 i think.
You just need a game that uses the source; so HL2, EP1/2, Portal, TF2, CSS, etc.
If u have Counter Strike: Source and then Install Garry's Mod, then you will have access to Counter Strike Source Content AND Half Life 2 content. But to have access to other Source games you will need to purchase the Game e.g Having Counter Strike and Team Fortress 2 will give u access to their content plus Half Life 2. But you won't have access to content from other Source games; Portal, Day Of Defeat.