You can try and spray a little lube in the key hole and try pushing down on the cap while turning it. Unless the gas cap assembly has been removed before, taking out the allen screws will not work as there is a screw under the cap itself to keep it in.
Tap on the top of the gas cap with a screwdriver or small hammer. Notice I said tap and don't hammer!!! It will loosen the cap enough to help unlock whatever is seizing the lock inside the tank. I had this happen on my 2008 ZX10R and replaced it with an aftermarket gas cap not requiring a key. The inside locking mechanism can get seized once in a while. Do not turn real hard because it will break your key off inside the lock. Keep tapping lightly and it will come... Good Luck