itch.io

Belated, but the DLC takes the form of some zipped folders, which you just copy and paste into the correct location in your existing installation.  So far as I'm aware, it shouldn't matter if you buy / download those folders from itch or GOG.

I know you can opt out of the ice physics, at least.  One of the traders in Bergen sells boots which let you walk normally on ice, which makes the jumping in the icy puzzles a lot easier.  (They don't work in the optional monk trials, though).