I have witcher 3 on GOG and using HeroicGamesLauncher to get it (Linux).
I had it uninstalled so I reinstalled it after the remaster release, which was released or I got it at version 5.00b.
There was a second update almost immediately, which was ~25GB in size through heroic. Also checking GOG, the update from 5.00b to 5.00c was same 1x1mb and 8x4GB chunks. But the update from 5.00b to 5.00c in steam was ~500MB. What gives?
The funny part was that when I did the update, I (or Heroic) absolutely borked it. First the download speeds was slow, second it would keep on getting stuck. If i paused and unpaused it would start from 0 again, same with restarting Heroic. I even tried to downloads the independent chunks from GOG and that didn't work, it kept on saying install not found even though I made sure the prefix and the install location were correct. So in the end, ended up reinstalling the whole ~60GB.
Come 5.0c-> 5.0.11, I was very scared. And low and behold, I ran into same problems, it froze and borked. I was almost going to do the same uninstall and get fresh 5.0.1. But I decided to try to first freeze the game version to 5.0c, the run verify and repair. Then ran and update and finally it worked.
What a pain.
Now I know it's not a Heroic issue but a GOG issue (probably) but just needed a place to report/vent.
Also hope this will help somebody not just on witcher 3 remastered but for other games.
Although not sure if its better than actually reinstalling the game as repair verify and repair also uses internet. But in my experience doing this, i.e. fix it to the previous version, repair, and upgrade, seemed to go a lot faster than the whole reinstall.