Yes, you do. You need steam and a source game such as Counter-Strike: Source. Or Half-life Source. Day of Defeat Source
Buy Garry's mod 10 off of steam.
technically yes, there were a few updates it is still called Garrys mod 10 though
Garry's Mod 10
no because garrys mod requires that you have a game that uses the source engine
Nope but it is useful, for example when you are using thrusters
Garry's Mod has now updated and includes all features of 13 Beta
Garry's mod tornado
No, but you need .lua to make your own addons.
Steam
yes
Not specifically. You need a VALVE game to play Garry's Mod, one of which could be Half-Life 2.
You do not make an account on Garry's Mod. You need something called Steam. http://tinyurl.com/2vwrgf5