This is a long shot but I have to ask, are there multiple saves for each game in the cloud? Here's my situation: my PC overheated during a game, while it was saving, so the save was corrupted. Not realizing this had happened, I launched the game again after my PC rebooted. It informed me my saves had been corrupted and wrote a new one. So I quit the game... and GOG Galaxy of course syncs my saves. Now the only save backup I can access is the fresh one. 15 or so hours down the toilet.

Is there anything I can do?

You can try opening a support ticket. GOG Galaxy only lets you download the most recent set of cloud saves. It's possible there's nothing else stored server-side.