Today my Internet connection started acting weird. After inspecting my traffic a little closer, I noticed that the process "GalaxyCommunicationService" was eating up my entire upload, to the point where I could not even establish a connection to any website while Galaxy was running.
I have no idea what it could be doing. The client starts automatically on start up, and I haven't played any GOG games in weeks, so I cannot really imagine anything regarding cloud saves.

Any ideas what's going on?

PS: it starts tanking my connection only after being up for 5 minutes or so, before that it does nothing.

Firstly, have you done a good old restart? If not then do so...

Now, I'm not sure when this was released, but today I'm getting an update for Galaxy, if it was released today and you have just installed it, then it could be a bug with that update. Now, you say upload, so this must mean galaxy is sending something back to server? Look in settings, and see if you can turn off bug reporting. I do not recommend leaving this off, it helps everyone when it is on, but it may be trying to upload a bug report back. Does it work now?

I'm sorry if I cant help very much, I'm not an expert, but that is what I can think of at the moment.

I had the same problem ...

If it is already tanking my upload, i can't change anything in the client. I did have to restart the client to make any changes.
And after I disabled the cloud for Witcher 3 the problem didn't recur.

For me this seems to happen after every restart of windows - although now it does stop the upload after some time. But I haven't even played any game in my GOG account for at least a week, so it should not be uploading any new saves.

I have this same problem, as far as I can tell it's been happening since the update to Galaxy that added cloud saves and the options to toggle features on and off. I figured it may be uploading all my existing saves to the cloud so I left it on overnight but GalaxyCommunicationService was still using my entire upload the next morning (I live in the UK and my area doesn't have fiber yet so this is a big problem!). I've set a bandwidth limit in Galaxy but since that has made no difference I can only assume it only applies to downloads.

I've just tried disabling cloud saves and bug reporting and restarted Galaxy and that seemed to solve it, then re-enabled bug reporting and restarted again and the issue didn't come back so I can only assume this is cloud saving doing it.

I have these games installed with save files present:
Divinity 2: Developer's Cut
The Witcher: Enhanced Edition
Divine Divinity
Neverwinter Nights Diamond
Anno 1503 A.D.
Darkest Dungeon

Of these only Witcher, NWN Diamond and Darkest Dungeon show in Galaxy as having saves backed up to the cloud. I've disabled cloud saves for them all and then re-enabled one by one, and it appears that Witcher is the culprit. I'm not sure if this is just due to file size or if there is something else going on here but hopefully this information it useful to someone. Let me know if there's anything else I can provide if a dev/mod sees this or something! :) I use cloud saving on Steam with no issues so I can at least say that it's not just that my line is so bad it doesn't support uploading like this.

Fucked up my Rocket league ranked match

I'm a bit late to the party.
I have the same issue as long as I keep a bandwidth limitation in parameter.

If I set no limit, galaxy communication is quiet.

Same issue for me. I've deleted Witcher 3 cloud saves, no idea what else it could be uploading. Makes Galaxy unusable with low upload bandwidth!