The 2 Gears of War and the two halo games are still the best selling exclusive games on the HD console. Even if they are losing their thunder, they are so far ahead of every other game, that they still will be the biggest hits for the 360. The only exclusive that is even close to them is metal gear solid 4 which is ahead of ODST. Halo 3 has sold better then the most critically acclaimed wii games, with only "wii games" being able to pass it, on a console that most "gamers" buy solely for the Nintendo games.
One thing that has happened is that most of the best selling games have become multiplatform. CoD, GTA, AC, GH, have all outsold gears and except assassins creed halo. Final Fantasy, and Battlefield Bad company also have a shot at outselling gears in total sales. There are others as well.
The two franchises might have lost some thunder, but they are still the most relevant games the 360 has, and PS3 still has no exclusive to match them purely from sales perspective. GT5 will probably be the first exclusive to pass Gears.