Linux

I have no idea what problems you are hitting or seeing, so I think that it'd be hard to offer much advice outside of long-shot guesses.

I've used xdelta, albeit not for GOG games, and didn't know that they used xdelta for updates. I mean, it works, given a valid patch file and the right source file.

Are you sure that the source file is the correct version? What output are you seeing when trying to run xdelta?

Have you looked into asking the cs.rin.. forums? I feel that would be a good place to get support

Heroic is working on this feature and you can try it out if you want to currently it just isn't deemed stable. Here is the PR: https://github.com/Heroic-Games-Launcher/HeroicGamesLauncher/pull/3020

gog galaxy through wine is not an option...

That's the primary way I install and play GOG games. It's easy to set up using Bottles. Galaxy used to run horribly on Wine, but it has improved recently. I help maintain the Galaxy installer in Bottles, and earlier this year we increased its grade from silver to gold, meaning all functions work with minimal glitches now.

It isn't perfect yet, it lags for about a minute right after launch, and I'd recommend going into settings and disabling the "Cloud saves" and "Overlay" features as these can cause crashes sometimes. Other than that, everything works well and performance is good.

Have you tried minigalaxy? I think it can handle updates.