Try using an old keyboard (PS/2) and pressing DEL.
It's strange not letting you in now after you've installed windows.
Try clearing the cmos, remove the battery overnight and replace in the morning and try again.
Try disconnecting the HDD and see if you can get into the bios without it rebooting.
I think maybe you've changed something in the bios or it's got an error or something, i'd remove the battery for atleast 12 hours, gigabyte boards hold power for a while so best to totally clear it and try again, might get an error on the first cold boot but should fix it.
Check event viewer once your booted into win7 and check if there's any errors appearing there.