You're supposed to be able to, but apparently people are having trouble with crossplay.

They keep saying "just buy the Steam version." I alsmost did myself, but I ended up buying it here on GoG yesterday out of pronciple. I want to support the DRM-free platform, end of story.

I just wish more of the game's base had that same idea. That they could migrate here.

OR. This would be awesome, and feasible. I hope the devs consider it.

The Wticher 3 team let everyone who owned the game on Steam migreat it here for free. I only played a couple hours on Steam before migrating my install to thei platform when they let me. Maybe these devs will consider that because of the crossplay and server hosting issues between Steam and GoG. I hope.

I doubt it since you have the choice of where you buy the game from.

They did do what you suggest for KS backers because the KS promised a DMR free copy of the game. I moved my Steam one over to GOG when they gave that option.

I succefully managed to play with 2 friends who both had it on Steam.

On some days it required some fiddling (restarting the server mostly), but overall it worked ok.