There might be a problem with one of the backup/offline installer files. When installing I get an error saying the source file texture.cache is corrupt. I download all the installer files again (because I don't know which bin file texture.cache belongs to) from GOG and the same thing happens. So I do some investigation and find out that I can uncompress the bin files with a tool called innoextract. This is the error it spits out which matches what i'm seeing installing it the normal way.

- "app/content/content0/jp.w3strings"
- "app/content/content0/kr.w3strings"
- "app/content/content0/pl.w3strings"
- "app/content/content0/ru.w3strings"
- "app/content/content0/shader.cache"
- "app/content/content0/soundspc.cache"
- "app/content/content0/staticshader.cache"
- "app/content/content0/texture.cache"
Opening "setup_the_witcher_3_wild_hunt_1.31_(a)_(9709)-2.bin"
zsh: bus error innoextract "setup_the_witcher_3_wild_hunt_1.31_(a)_(9709).exe"

The extraction of all files up until that point is fine.

Yup, the files are corrupt. Screenshot of the built in integrity checker attached. I've tried downloading this game multiple times using Safari Firefox and Chrome, and it's the same result.
As a test I tried downloading other offline installers and they're fine, it's just the Witcher 3 offline installer files that are corrupt.