CaBrA, Best thing to do is try the below demonstration to understand what I'm talking about:
1. Start Gmusic addon..
2. Search for any song you don't already have. Just highlight the song title (but don't play it).
3,. Then, press "C" on your keyboard (or right-mouse-click) on the song title to select, "Add to library" from the context menu.
Immediately, after selecting that... the Gmusic addon will display notification text saying,
Library Update needed (even though, that's what we just tried to do by selecting update library) . Currently, we have to wait a minimum of 1 hour before this song shows up under the local Kodi Music Library (if you have one setup already).
In other words, you wont see Items show up in your Music library unless you actually tell it to do that.
@oreverguest: I wasn't able to add Albums to my library in this fashion. It would be great to be able to add Albums too to our Library in the same fashion as above (via context menu "Add to Library".