Hi ! I recently bought the witcher 3. I played 2 hours on it then when I wanted to relaunch the game I had a error message so I decided to unistall it. But now I want to download it again but in the Gog platform the install button is in grey so i cant dowload it ... How could I download the game again ?**

I suppose you're using Galaxy? I don't have much experience with it, but what happens if you try to install it from the installer in the same folder where the version installed by Galaxy was?

You can download the installers of your games through this page: https://www.gog.com/account

If you click the button to the right, then expand the Manage installation submenu, is the Verify / Repair menu item available?

Try to close and restart Galaxy. I mean actually restart it from the tray.