How much fiddling have you done with it? Try keeping the key turned, and try turning it different amounts, both forward and backward.
My first reaction would be to twist the crap out of it and/or drill it out, but those are probably both bad ideas.
I’m assuming the key is turning the tumbler? Just keep fiddling for a bit, you’re probably just not lining up the tab with the slot on the other side quite right.
Here’s what worked for me. Grab a pair of Channellock pliers (also called pump pliers or arc-joint pliers.) Adjust it to get a good grip over the cap, and pry up until the locking part snaps off. The inner part will unscrew just like a non-locking cap. Then you can go buy a new cap.
I had to puzzle this out once when my evil dad lent me his car to drive from Bradenton to Mousetown without giving me the gas cap key. :smack: When I got back, I handed him his car key “and your new gas cap keys.”
My eternal thanks to Santo Rugger, FatBaldGuy, bannerrefugee and especially AskNott for your sage advice.
The channel locks worked. I am now going to the gas station to add my pain and suffering to my fellow country men and women. God have mercy on my soul…and wallet.