You can't use credit in conjunction with a voucher, I believe. Which is kind of bullshit to be honest...I was a big fan of GMG at first but I haven't actually bought anything from them in some time.
I never have credit so guess I haven't noticed. But even so, those continuous 25% off codes they run usually still make the game cheaper than anywhere else, not counting the shady russian key sites that aren't really a road you want to go down anyway.
Even at the full $50 the game is a no-brainer buy because it's excellent. So if you can get it for $40 with a code that's still a bargain.