GTA series is becoming pretty irrelevant and boring. Halo 3 would have outsold it by a mile had it released this year. That was obvious. No doubt Halo is a much bigger series. Gears of War 2 would be a bigger seller also.
Also this rumor is obviously not true. GTA cant be Sony exclusive without losing most of it's sales since 360 is the dominant console in NA and Europe and Japan doesn't buy GTA.
But I'm just saying, even if Sony gained GTA exclusive, it wouldn't matter much.
BTW this thread title is pretty misleading since it's obviously not even much of a rumor from a small PS3 fansite.