ProtonDB says the GoG version is totally broken right now. Will likely be fixed soon. Larian is really committed to making it playable on Linux.

Game is an absolute masterpiece on top of it all. This kind of game needs unequal levels of support. They knocked it out of the park AND they did it all in the right way. I’m encouraging people to buy it and play if they like RPG, tabletop or even just interested in forgotten realms. One of a few games I’ve purchased for full price on Steam.