Just a good hard pull and the door should pop off. The filler neck and actual spin on gas cap stay in the car(actually, you should remove the gas cap to get the door over the filler neck, but you get the idea). The door is stuck to the filler neck with just a rubber grommet. Out is easy, getting the grommet back in place when putting it back together is the tough part. There’s a trick to it that escapes me at the moment.

Thanks. I see what I need to do now to get it out. I’ll use a hair blower to make the grommet a bit more pliable before I try and get the door off. Still at bit cool here in NJ.