Is it %appdata%\StardewValley\Saves?

Because that folder is well under 200 MB on my computer, but Gog Galaxy is throwing a sync error, claiming it is over 200MB.

Apparently, it also saves the screenshots folder (WHY?)

Cleaning it is a pain, if you ask me.

Close the game, turn off syncing in Gog Galaxy, move or delete the folder on the computer, delete the files on Gog Galaxy, only available through a website link, no way I could see to get to it from Gog Galaxy.

Real dumb system.

I see no value in backing up those screenshots in the first place.

https://www.gog.com/account/cloud-saves/page/1

is the place to delete the files online.

WHY? I delete the files on the computer, then on the website, then turn sync back on, and both repopulate from some hidden store!

What a pain!

You can try and delete them from the cloud directly. Check the link below
https://www.gog.com/account/cloud-saves/page/1