TIP: The original fuel tank cap is black, not chrome. What's on your bike now?
The key code is definitely stamped on the backside of the seat lock. You may need a magnifier to see it, but it's there.
If you can get Kroil penetrating oil, douse it well and wait. It will likely get it working again.
The GPZ came with two keys; one for ignition and one for seat locks. I assume the number stamped behind the seat lock will be related to the key needed for the said seat lock.
However, I checked on my KZ750 that also has two keys; one for seat locks and one that works in igntion AND fuel tank cap. Based on this, I believe that, for the GPZ1100, it is the ignition key that should open the fuel cap. So, far, I tried both keys. The key turn a little but not enough to unlock the cap. I could force a little bit more but I'm afraid to break something. All the time, I'm strongly pushing on the cap itself while turning the key but no luck.
That being said, I believe the fuel tank cap barrel key is not stuck and it is possibly the spring loaded locking piece (shown at upper portion of Martin's second picture) that could be stuck.
I did not work on the GPZ so far and I could have a bad surprise (something like a brownish liquid filling the tank) when I will open it. When I'll give a look at the GPZ, I'll first try to push out the pivot pin of the cap. If it doesn't work, I'll remove the tank and try to work from underside.