I noticed that my GPU hits 75 degrees when playing. Should it be going that high? Frostpunk doesn't seem that graphically demanding, so I don't know why it would affect the GPU that much.

This game will run my GPU up to 190* F if I dont have my case fans set to high. It may not look like it, but this game IS demanding on your GPU.

Yeah, might be an optimization thing.
I dropped the framerate down to around 40 and it's a lot more bearable.
Some games flat out ignore VSYNC and have crazy frame rates up in the 200s. Necrovision Lost Company is one just game.

It does not ignore Vsync settings however there's a lot of particles, especially smoke, in the later part of the game that is quite taxing. Try reduce particles quality in graphics settings, and shadows / global illumination if that isn't enough.