TheBlindBatman: The cloud save system is something handled directly by Larian, and I've discovered that it can be tricky to make it work on Steam Deck from the GOG version, but it is doable.
I have installed my GOG version of the game through Heroic Game Launcher.
Then, I have added a shortcut of the game to steam (directly from the option available in Heroic Game Launcher).
I don't think this is mandatory, installing the game and lanching it by other means should work, as long as your gamepad is detected (see next part).
Finaly : I have started the game from Steam
in game mode, the important part is to
use a gamepad, so you can see on the main menu of Baldur's Gate 3 a input tips : "X to connect to Larian Account" (not sure about the exact wording but you get the idea).
Once logged, you still need to enable "crossplay save" in the "general" section of the option (near the options ytou can set the number of auto-save). After the option has been enabled, you may need to wait few seconds while the game download your cloud saves in the background (there's no indicator, just wait one minute or two).
Now, you should be able to load your cloud saves synced in your Larian account.
That works, but the connection does not stick inbetween executions: You have to reconnect to larian network every time you launch the game :((( (Other people
Given that the credentials are not saved, it's really awful to have to type them in everytime you want to sync your cloud saves... Do you have any solution for this?