If the granite was supposed to be cut at say the 26" width all the way, then tell the installer to go back and cut another slab. You can take out part of the drywall, but it sounds like the mistake is on his end. I have seen granite slabs that were cut straight but the wall had a hump in it. So they would notch the drywall in the spot where it stuck out. But in these cases, the wall was causing the granite to show a gap behind that was less than 1/2". When they took out the small bit of drywall, it was flush. The wall being out is not their fault. A slab that is an inch off is.