Had the same problem, so here what I did.
Find "setup_the_witcher3_wild_hunt-1 " to "setup_the_witcher3_wild_hunt-7" on your discs and copy them manualy
(just drag and drop) in your designated path. For me its: My coputer<Windows<GOG games<Witcher 3 Wild Hunt.
Then just start "setup_the_witcher3_wild_hunt" that has a GOG lunch icon next to it.
Download patches and you good to go
Hope this helps.