if i pre-order a game from GMG can i redeem it as soon as i get the code?

You have to wait for the release date. You can't play an un-released game.

Depends. I got an amazon key for Scribblenauts and it's on preorder status But it reverted to it because I could play before, so I dunno.

Many vendors that provide game keys will only send them upon the game's release, though I don't know if that's GMG's practice. You might want to check with them.

Borderlands 2 gave me a key about a week before.

(via gmg)

Sometimes GMG will have codes before release allowing preload, sometimes after it. Depends on publisher, I guess.