Breja: Well then, I guess I will not know what you're talking about. I have absolutely no clue what the OS or new PC has to do with anything... search for the game you want in your library, download the installer (if you don't already have it backed up), install the game. Simple as that, other games in your library have nothing to do with it. >shrug<
joelandsonja: I have over 500 games in my library. Manually installing each one would take me days or weeks to accomplish on a new install. This is why most people use launchers and front ends. The problem is that launchers don't differentiate between games in your library that you want, and hidden games, therefore you have to sift through a bunch of random crap when trying to install your games.
Honestly, I think you're aiming for the wrong solution. GOG lets you mark an item as hidden, which makes it disappear from your library, and that already seems the feature you're looking for. You say it doesn't work for you when using launchers because they don't recognize this hidden tag... so you're asking GOG to implement
another way to make items disappear from your library.
If the issue is that launchers don't recognize hidden items, then the solution should be for them to recognize which items are hidden, shouldn't it?
I'm not sure why they ignore the hidden tag, so I don't know if it's GOG or the launchers to blame, but that's where the fix should be.