GOG Galaxy stopped working so I tried re install it. I followed another thread and I tried to install it and it keeps on giving me that the same installation has been installed. I can't use GOF Galaxy because of the mess up.

Uninstall before reinstalling. Install this older version 2.0.37.384 if you're not using Windows 10. And delect these folders
C:\ProgramData\GOG.com
C:\Users\yourusername\AppData\Local\GOG.com
if they still exist after uninstalling.

edit: you might receive an error saying unable to read your Galaxy databae after launching the older version you just installed, it can be solved by adding games manually to your account.

i deleted the first programfiles/gog and the other you suggested and nothing seems to work. still getting message saying it already has the last major build.

You sure you uninstalled it by using unins000.exe in your GOG Galaxy folder or through Programs and Features in Control Panel? Because it seems you didn't uninstall it properly, the info left in your Registry or files remaining on your disk might lead to this prompt in my point of view.

And by the way, C:\ProgramData is a folder hidden by default, not this one: You have to go to Folder Options in Control Panel, check Show hidden files, folders, or drivers in the View page. I didn't delete either of those folders, because the uninstaller deleted them automatically in the progress of uninstalling. I got that suggestion from here and I told you just in case they still existed after uninstalling.

Do not update to the latest version, which is 2.0.40.1 and provided by GOG atm, because it has compatibility problem with any operation systems before Windows 10.

I deleted the folder GOG in Program Data and i deleted it in User files, I will try to uninstall by the old fashion method.. Also my OS is windows 10 but was upgraded from windows 7

Got a fresh install of GOG Galaxy working. Now I have to install my games again.

Took me a while to figure it out but this is what to do if you uninstall a game and it still shows as installed with a greyed out play button:

Simply uninstall GOG from the control panel and reinstall it. Now the game will not show in installed now. You are all welcome! Happy gaming!