It won't beat the COD games, but it might be the best selling exclusive outside of Halo.
Gears is a bit of an unknown quantity in the current generation; Gears 1 Ultimate did well, but a remaster of a classic and a brand new game are two different ball games. The X1's smaller base compared to the 360 could be a hindrance, but I think the bigger danger is if it rubs longtime fans the wrong way, like 343i's Halo games did versus Bungie's. The Coalition has some big shoes to fill.