You only need to activate the game one time. Redeem your key here: https://www.gog.com/redeem (If you bought it as retail version or from Amazon or humble store).
If you bought it here you don`t need to activate it.
Then go to Account - Library - Witcher 3 and download the installer and patches with your browser.
Or go there and use the GOG Downloader (links can be found behind the more button).
Or use the back up feature of GOG Galaxy to get the installers for a later offline installation.
Copy all files on a flash drive or an external hard drive and install the game on your offline PC. You don`t need an internat connection to play the game and even GOG Galaxy is optional.

Many thanks for information...

Have a good times :)

you can even copy the already installed game onto another hard drive and use it there. even if it's a different OS.