This is a major disappointment.
Galaxy doesn't fit the need, because I don't install games on my internet machine, only on my in-house LAN. So it's offline only.
Which is something of a problem because I have no idea when there's a game updated what exactly changed. Changelog doesn't really help. Take Train Valley, which says it's been updated, but the changelog says the last change was on 1 AUG 2019. I'm left with downloading EVERYTHING, then using a file sync program to figure out whether I should overwrite the old content or if it's just duplicate files with a new date.
I am a bit confused. GOG Downloader also did not show what had been updated - you still needed to look at the website and follow the "What did just update" thread (if you did not want to use GOGrepo or similar tools.