I have historically purchased GOTY edition of the Witcher 3. Now I decided to play it again, so I logged into GOG Galaxy, updated it, logged in and went to my library. The download went fine, but now I am stuck with a game that says "Ready to install", with no options available.

I tried running GOG Galaxy as an administrator.
I tried logging to Galaxy via web browser and downloading the installation file (small .exe) from there. I ran it as admin as well. All it does is to open GOG Galaxy and leave me hanging, with the game "Ready to install".

Check out the screenshot.

How do I install it?

Since the screenshot did not upload, and I cannot post links... Here's my view:
i . imgur . com / X4qZ8ts.jpg

Maybe, uninstall Galaxy and the game(from control panel), re-install both.

...and re-download 40 GB of game files. Ugh... Will try.

Galaxy is a pain in the ass. Be rid of it.

Any DRM-free designated game, by their publisher/developer, does not need its .exe to be authenticated by any client, prior to or at its launch. And that means that ,the Witcher 3 series does not need any DRM/client, including GOG Galaxy (which is not a DRM), to run prior to or along with it. Their .exe can be run by their installation file just by double clicking it, if the game is indeed fully downloaded and updated by GOG Galaxy.