Last time I ran into this was decades ago with a frozen external locking gas cap on a VW Vanagon.
That time I had to go in and buy a large hot coffee to pour on the cap to thaw it.

Today - fortunately I had a hot coffee in a thermal cup with me - because this grocery fueling island sold NO coffee!
The covering flap - I banged on it to drop the ice - then I find a block of ice surrounding my gas cap!
We've had freezing rain, and 4.5 in. of snow - and temps in the teens for highs....!
The snow squeaks when you walk on it....yeah - that cold.
I barely managed to thaw the cap enough to refuel my Chevy Silvertomato 1500 today.
Bit of a mess - but most of the refueling area was a sheet of ice. Snow not removed, ice not treated.
So...I added a coffee to the salad.
Look at the spike of ice through the flap.

I had to carefully poor very hot tap water on the door seam to get the door open.
I'm ready for spring!

Stig