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It's rather strange that there isn't such a possibility. Other games on gog sync my saves normally. I checked in the game configuration and I can't select the option to synchronize saves.

Is GOG going to add this option in the future?
If you create a Larian account and sign-in while playing, you'll have cloud saves.

I didn't create a Larian account myself yet, but I understand that cloud saves were disabled right after launch but were enabled again as of the latest patch.

About whether GOG will enable their own cloud saves feature, we don't know yet, but I certainly hope so.