So.
I have been trying to install witcher 3 via "Gog Galaxy" but its stuck at 100% and says "installing", the "play" button is greyed and im unable to play. After this I wanted to try installing it via Gog downloader but I can't find any guidance or help with it. Any suggestions? Every piece of information is greatly appreciated!

You cannot install the game via the GOG downloader. The downloader is just what it says... a downloader. It is not a game client like Galaxy or Steam. You download the files, and then, once everything is completely downloaded, double click on the installer .exe file.

THANK YOU IT WORKED!!