This is how cloud save works on the mac version on GOG:

Sadly there is no normal GOG cloud saving available for this game. Instead the cloud saving is handled by Larian/the game itself.

If you start the game via GOG, the larian launcher is started. Here you should find a registration/login form for a Larian account. After you created an account, there is a checkbox in the ingame settings, where you can enable cloud save.

But the launcher is broken at the moment and only shows you the "Play" button and is missing the whole login/reigstration form.

If you know how to edit a text file and use the terminal (maybe you need xcode, too) you can fix it yourself following these instructions:

forums.larian.com/ubbthreads.php?ubb=showflat&Number=928296#Post928296
(Links are not allowed at the moment...)

Short summary:

In your terminal:
open /Applications/Baldur\'s\ Gate\ 3.app/Contents/Info.plist

Find CFBundleDisplayName and add BG3 as a string like this (only if it is empty, otherwise this fix is not for you):
<key>CFBundleDisplayName</key>
<string>BG3</string>

After you've saved the file you need to resign it (maybe scode is needed for this step):
codesign --force --deep --sign - /Applications/Baldur\'s\ Gate\ 3.app

Hope this helps since I was googling half a day to find a fix.

cloud save is the Steam server and works on Steam copies of Bg3 only... Gog does have a save server they could easly use to do this but Larian decided against it and Larian controls things for now

cross saves are the Larian server and while it did work during EA the gog sync has not worked since the PS patch months back

as a stick your finger in the wall option Larian decided to remove the tick box option from the UI which confussed the shit out of everyone [mostly because they didn't tell anyone why] and people started posting how to hack the UI to put the tick box back

Larian then decided to turn the tick box back on in their UI which is great but it doesn't actually work which is to say a save file never arrives at Larians servers so puting a tick in the box is not relevant

Had the issue as well and reset the launcher. Afterwards it worked fine. I can login to Larian Account and it syncs my saves fine without any modification.