Ok, I'll update this. I decided to put the metal block (with the gypsum layer on) into the oven at 250 °C, to see if I could convert it to β-anhydrite, then see if it could be more easily removed. I then carried it over to, and placed it in the sink, sounded like it was going to fall through the sink
, and cooled with water. The gypsum was then much easier to remove by hand. I'm thinking it lost its bond with the metal surface as it heated to β-anhydrite. But not sure.
If there are any ideas for other chemicals to remove please let me know because it was a bit dangerous to do what I did with the heating (and transfer) of the block to sink.