This happened to my friend and we found that if you take Compressed air and spray it into your 360 that it may work again, but, about 2 weeks later he bought a new 360 so I am not sure if this is efficient

this is true

gears of war 2 is brutal compared to gears 1

If none of the above helps, try your disk on another console to see if thats the problem (if you have access to one) Otherwise try this maybe

If you have a Hard Disk Drive (HDD) unit or Memory unit (MU) attached to the console, turn off console and remove the HDD unit and any attached MU, turn on your console, and then try the disc again. If this seems to resolve the problem, turn off your console, reattach the HDD and MU, and then delete any saved games for the game that is causing problems
Quoted from http://support.microsoft.com/kb/909819

Copy and past that site into your adress bar and try some of those steps. One might work. We could probably tell you better if you posted again if the previous steps didnt work, or if they did.