Hello all!
I had some problem with the game so I uninstalled it using app in game folder on my C drive- not from GOG interface. Now the game shows as installed, but since there are no files (i deleted everything, including saves) I can't play - the option "play" is grey and doesn't click. So, GOG tells me I have this game installed (it;s on the list) but I cant play or unistall it. What options do I have? Thanks in advance for help.

Check your installed apps to see if it's still in there. If so, try uninstalling it from there. It may remove some leftovers that way.

If GOG interface means the Galaxy client, there's an option to verify/repair a game installation in it.

You can download the offline installers, either through Galaxy or your browser, and use them to install the game.
Install to the same directory as the previous installation was in. It might make it playable through the client again. If not, you might want to let the client search for installed games.