So I've been playing and saving and stuff and racked up almost 780 MB of save files while apparently the cloud only supports storing 200 MB of save files. Fair enough... problem is somehow Galaxy uploaded all the 780 MB just fine all this time. Now if I delete the saves locally (after backup) to reduce the size of the data, it just syncs everything back down when I start the game. Meaning in effect I'm locked out of ever using cloud save sync with gog again. I've noticed the only way to delete save files for sure is when I do it in the game. I don't want to do this for hundreds of save files though. Is there any way I can clean out the cloud storage and upload my last 10 saves or something?

I just encountered this for the first time last night.

I think we've got 2 options:
1\ like you noted, we can delete from within game
2\ visit https://www.gog.com/account/cloud-saves/ and either individually delete saves from cloud-saves directly (or click 'remove old saves' button and have it try and intelligently do it for us)

Either way, I'm taking a manual backup of my games save folder before proceeding. May disable cloud save entirely.
I would say we need to first create our own backup of the whole saved game folder (or not - personally I will be, but each to their own :), then go in-game and delete. Seems to be the only reliable way to do this.

I'm just reading about this now:
https://support.gog.com/hc/en-us/articles/18730340487709-Review-your-Cloud-Saves-to-avoid-loss-of-files?product=gog

From the article it would seem that there's always been a 200mb limit, but it's never been enforced? Now not only is it being enforced (galaxy wont sync if the folder is >200mb) but it's going to delete excess files if we don't manually take care of it by 31st August.

What a PITA honestly, space could scale per-game or per-account. I played for all of about 15 minutes last night and generated 36MB of new save files without trying. At that rate, given my average play sessions are typically hours long, I'd be lucky to have cloud saves work for a day.