Thanks. That code is listed on the Steam forums along with a list of GMG games that activate on steam:
http://forums.steampowered.com/forums/showthread.php?t=2046945
I don't see Sleeping Dogs on that list, but it's 10% off on GMG right now, or 30% off if you use this code (in theory, I haven't checked to see if you can combine the discounts).
EDIT: On the
Sleeping Dogs page on GMG, it specifically says the DRM is Steam and that you need a Steam account to play, so that solves that...Also, I've confirmed that the discounts stack. Comes to $36.00. Sold for me. (Or you can grab Darksiders II for $40.00). Also FYI, as OP says this is for ONE GAME, the code only won't apply to an entire cart.