I think of the games we know coming next year that Zelda:SkywardSword and DukeNukem:Forever both have a chance at outselling Gears3. But its the games we haven't heard announced yet that pose the biggest threat to Gears. Say the next Modern Warfare game which will probubly see release next winter.
As for Pokemon Black and White being split and calculated seperatly. Thats ludacris they are the exact same game with very minor differences. They deserve to be credited as a single game or at least ports of the same game. Also we have no clue what the 3DS will have instore for next winter, with such a powerful lineup headed early next year I doubt that Gears3 could hold its own against all of them.
Also don't forget Halo, 343 said they expect to release a new Halo each year. Considering they have had almost two years to develop a Halo product I'd say a new Halo game would also challenge Gears3.
Gears3 will be in the top ten releases in 2011. But I don't think it will even make the top five if you include competing titles from Wii/DS/3DS, PS3/PSP2 and 360.
Its a good game , I played it after all. But it isn't first place good