Stebz: Yeah I am having some of the same issues as you i think. My game is corrupted to hell right now so the npc's are constantly bugging out and the geometry around buildings and cliffs is dropping off into holes of oblivion. Tried literally everything the devs and GOG support have suggested, but can't repair the game because of the stupid "Server Problems". Been spending my spare time out of work trying to fix this damn issue but after 4 days, I am starting to get a bit pissed.
Hopefully someone finds a solution.
Dude the GoG Galaxy servers are fucked. Don't bother any more. Might as well remove it from your system
You're be better off just downloading the whole game and the patches from the gog website using GoG downloader. If you have GoG Downloader installed but haven't figured out how to use it, here's the how-to: Once you've logged into the website in your browser, click on Account and select Library in the drop down list. This will take you to your game collection. Find your Witcher 3, hover your mouse cursor over it and you'll see a small button marked with a downward pointing ^ sign. Click on it and select View Downloads. You can now see all the downloads available to your game. Click on the button that says More, and select GoG Downloader Links in the drop-down list. In the page that ensues, every link you click on should automatically activate GoG Downloader when download starts
Hope this helps. Took me a week to figure out. Good luck