The option can not be activated.

1st yes it's possible but gog does not have it yet and Franky you don't want it because cross-save does the exact same thing

Is it though? I have cross save on, for both my Game PC and Laptop and saves don't seem to be available across the installs.

It used to work fine for me, with the last 5 saves uploaded (so if you play with the character creator and then start an actual game, where you can have 25 quick saves and 25 auto saves by default, you will not be able to sync your other characters without loading them back and saving, but that is not too much of a hassle if you do not constantly switch).

Since the latest patch, the game keeps saying it is syncing when saving (with the Load/Save button disabled while "it does it"), but no saves are marked in sync (there was a cross to the right of the save name), and switching computers, there is indeed no more sync.
So meanwhile, I sync them manually ...
But as I have not read about this issue anywhere, I am afraid there is something triggering this behavior that I have yet to find.

it doesn't work right now no, Larian sign in server went down about 2 hours into the launch [also taking out their forums and muliplayer]

right now the cross-save is out so make back ups or just start a new character when you change machines... note Steam players are lossing their cloud saves and we still have local so yippee for Gog

Cloud saves aren't working as you said.

This is really disappointing for me.

I will ONLY play this game via GOG and I frequently use 3 different PCs, but I'm forced into a crossplay save that doesn't work.

for ref your saves are at;

C:\Users\%Ussername%\AppData\Local\Larian Studios\Baldur's Gate 3\PlayerProfiles\Public\Savegames\Story

just copy them onto a Usb stick and paste across when you switch computer systems... its safer than Larian and less stress on your network

Is a Larian account needed for cloud saves? I am seeing the attached error (I did not sign up for an account as I am playing single player).

If an account is needed then I think that should be called out loud and clear on the store page. At the very least is there a way to turn off this error?

The Larian launcher cloud saves are what was broken. They turned them off while they fix them.

Steamcloud works and was never the issue.

GOG, for whatever reason, has not enabled cloud saves for this game (yet).

Or just share the above folder on the network, then copy each time*.

*making a batch file is even less work
@ECHO OFF
xcopy /s "bg3 network path" "bg3 local path" /Y
cmd /c ""path to bg3 folder" -f "path to bg3 program""

Guess I can stop stressing out about it, then.

I had sideloaded this into Heroic to fix an incomplete graphic issue I had Saturday downloading it from Heroic. The saves were syncing at the time. But after I sideloaedd my PC install and got the graphics working right, the saves would not sync at all.

I figured a shutdown on Larian's side was possible, but kept trying because I want you play this on lunch breaks.