djp42: Suppose I reinstall Windows 10 due to a CPU/motherboard upgrade and I have my GOG purchased games installed on my "Data" SSD, if I run the installers for those games over top of the already installed games, would the installers just verify that all the files are there and just install the missing stuff that was on my "OS" SSD such as registry settings, DirectX files, and Visual C++ libraries? Or would the offline installers completely ignore what's already installed on my "Data" SSD and just reinstall the whole game again, overwriting the files, making it pointless to install those games on a separate SSD?
The offline installers won't acknowledge the existing files and will simply proceed to overwrite whatever is already on your disk. You'll usually get a warning that the target folder already exists, but then the installer will simply proceed to overwrite everything already in the destination folder (even if the files are exact 1:1 matches). In this case, you could simply link a shortcut to the game's primary .exe file to your Start menu or desktop instead of reinstalling the whole game.
If you're using GOG Galaxy, you should be able to manually indicate that you already have a game installed. Galaxy will then do a quick file verification and add the game to your library. This will add the game back into Galaxy, although you may need to manually add the game back to your Start menu, if you care about that.
Note that there are a few games in which the GOG installer does more than just dropping files into a folder and setting up a few shortcuts. For example, the installer for Diablo also configures several firewall rules, adds legacy DirectX compatibility settings to the Registry and configures an alternate multiplayer server. In most cases, you should be able to run the existing executable without needing to reinstall. But if your game acts really weird, then try doing a full reinstall from the offline installer.
