The most impressive aspect of Gears of War is its graphics, so if you downgrade those you could in theory make a version for the Wii and also PSP. But then the focuss would be more on the subpar story telling, game lenght, bad acting, lack of varierty, etc. (IMO Gears overall is overrated, IMO due to its good graphics).
Other games like Lair IMO won't be possible by just downgrading the graphics and sound, there's too much going on at once in this huge virual world. It wouldn't be just an immense downgrade, it would have to become a different game altogether like many Nintendo DS versions of PS2/PSP games have become.