Hello!
I own Warcraft: Orcs & Humans both the Gog version and the original CD version, and I would like to have them both in library, but when I search for it to manually add (the cd version) it finds only the Gog version and it already says owned so I can't readd it or something.
Is there a solution for it?

EDIT:
I used Warcraft II: Dark Saga, since that is a PS game and will never find itself in my Gog library. Even though I guess people will see I played Warcraft II: Dark Saga, for me it is Warcraft: Orcs & Humans. It's good that it has the same company shown, edited the launch date, name and pictures, all it has wrong now is the game details where it says it is Warcraft II: Dark Saga, but I guess until Gog adds another way of manually adding games.. this has to do it.

It's possible. Use the arrow keys to navigate to your game and click "Mark as owned".
Look at the attached screenshots for further information.

Hello,
It seems that you can add duplicate library page by clicking on "Mark as owned" box in the search bar (with your game typed in). Then, at the top of the Library page, click on the icon with two overlapping squares and select the "Other" version of the game. Finally, you can link custom executable, and you have two separate games!

It already is marked as owned... It already is shown as owned... and I also want it as another game/title in my library.

why?

If I'm not mistaken, this is the only way to have multiple library pages of the same game in GOG Galaxy 2.0. So, if you have it marked as owned, then you should be able to link an executable. Also, you must use "Add games manually" option (e. g. through "Add games & friends" button in "Recent" tab). I tried, and already owned games can't be added through search bar (magnifying glass icon next to your profile). I've tried this method on several games I own on GOG and it worked every time.

Because for me they are 2 different titles. Oh, I get it now. But while I do am able to do what you say, all it does is make the game have 2 platforms. Other and Gog. That's not what I am looking for tho unfortunately. So I used Warcraft II: Dark Saga, since that is a PS game and will never find itself in my Gog library. Even though I guess people will see I played Warcraft II: Dark Saga, for me it is Warcraft: Orcs & Humans. It's good that it has the same company shown, edited the launch date, name and pictures, all it has wrong now is the game details where it says it is Warcraft II: Dark Saga, but I guess until Gog adds another way of manually adding games.. this has to do it.

Yeah, that's also a way to do it :D
This could be easily solved if GOG Galaxy 2.0 supported importing custom games into the library (which are not in the database). In addition, lesser-known games or mods could be added. I hope that one day we will see this feature.

Just use tags.