I would agree I'm glad this feature is here now, and it's nice and user friendly, but I really need it to be a bit more robust than this in the final build preferably. I would recommend maybe an Advanced button for doing more.
Not only should we be able to add entirely a game that's not available in the database, I like GOG but I also use a lot of Steam's services even on non-steam games. Such as the controller support and screenshot database, especially since I have a Steam Controller.
Since I have a Steam Controller, Steam absolutely must launch the game, non-steam game or not, or I can't use my controller, so I won't use it to launch those. Even for those not using a Steam Controller, Steam has a really robust ability to give you Alt-Tab to your gamepad (whether it's their pad or others) rebinding support for any game, and a desktop profile that can flip around and stuff so you could alt tab, look at a guide/discord or whatever and quickly flip back. These are really great tools, but if one wants to use these with GOG it's not really possible on non-steam games right now as Steam's overlay has to attach to provide those features. Right now I don't think anyone else has any pad tools this robust for doing desktop controls and all that stuff too so it's a really great feature to be able to support on things outside of GOG's own client.
That being said, the advanced manual adding I think should happen. Now in my use case, usually one would just use a Steam URL in a shortcut to launch a game Steam has set, which would fix this right up for me on non-Steam games. I was considering just setting non-steam games to "steam.exe -applaunch <number>" However, I don't think there's a way to save command line options. Without command line options I can't really make that happen, which made me think, oh right, we need command line options added! I don't think there's currently a way to do those. Further more, in my case especially I really would like a way to edit the launch command on imported titles as well, for the same reasons listed above really