After some fiddling in the first week I never really cared about getting my gog games to my Deck - but now for that I'd be interested what's the best option to install, keep up to date and sync my saves?
Definitely Heroic Launcher. It will do everything you want for your gog, epic and amazon games. Cloud saves are still an experimental feature (afaik) though. Oh, and updating isn't automatic. You'll have to update by going into desktop mode, starting heroic launcher and update your game from there.