According to Microsoft, Gears of War 2 had 1.5 million users on XBL play said game, over the weekend.
XBL's penetration rate is roughly 60% of the entire Xbox 360 userbase. If that is indeed true, you can use a simple formula to get near-actuals:
1.5 / 0.6 = 2.5 million units sold by the end of Sunday
So that'd put Gears at 2.5 by the end of Sunday - certainly more than enough leeway for what VGC has Gears at (2.1m).
Back from the dead, I'm afraid.