As I live in the backend of nowhere I have the proud internet speeds of about 500 kb/s. Go me! So I downloaded the game onto a hard drive at work and brought it home. I clicked import folder in GOG Galaxy and selected "The Witcher 3 Wild Hunt".

Error Installation Failed. Well crap.

First thought I thought it was maybe that I'm missing a file so I asked a friend to give me the folder properties so I could compare.

The exact same. Has anyone come across this issue, does anyone have a work-a-round?

Much appreciated.

You should try the import via the + on the top left corner of Galaxy and there select "Scan and Import a folder".

The same error occurred to me when I was trying to import the folder via the "More" menu.

Try this and tell us what's going on.

For me it was exactly the other way around: chose the game from the library, More > Manage > Import folder.