Problem seems to be the way how patches are applied here on GOG Galaxy. It seems to create a temporary backup copy of the game into a game folder before applying the patch. If the game is as huge as Cyberpunk, it can be time and resource consuming, depending on what is in the patch actually.
I found out when this update seemingly froze at 57% and my PC suddenly slowed down. So I checked and found out that my already quite full disk suddenly shrunk of ~60GB, and Cyberpunk 2077 suddenly occupies 119GB.
it'd be nice if they gave prompts like "creating temporary backup, please wait eta 4yrs". for example.
Alright guys, I know this is annoying af, having been through it myself. Here's how I did it:
1. Exit GOG Galaxy.
2. Go to Task Manager and make sure that anything saying GOG Galaxy is ended.
3. Restart GOG Galaxy, and the "download" should restart. If it does, don't worry if the download was already completed, it will detect it and go straight to installation. I'm ten minutes into a Galaxy restart, and I'm at 65% and counting.
I don't think that resolves the issue, gog still creates a backup bro.