There is definitely a chance. I really can't think of a single good reason why Epic wouldn't go multi-platform with Gears. It would literally be throwing away millions of dollars.
Even if Gears doesn't come, I'm almost certain Epic is developing something else that will be multi-platform.
However, I also expect Microsoft to ready with a new IP of their own next gen to counter-balance that. If they don't, then they deserve to take a hit.