Hi Jon. The secret is that apps have their own databases. So you need to find where your GNOME Music app is storing its data, and pass that location to tracker3 --database=.
For example I have gnome music installed via flatpak, so its data is hidden in ~/.var. With this command I can see everything it has stored:
tracker3 export --database ~/.var/app/org.gnome.Music/cache/gnome-music/db
If you have gnome-music installed via your package manager then the corresponding path is probably ~/.cache/gnome-music/db.
In the old days of Tracker 2, there was a single “Tracker database” which stored everything. But since Tracker 3, apps can easily create their own database. The filesystem index created by Tracker Miner FS is one example, and the GNOME Music database is another. I wrote about this a while back here - Tracker 3.0: What’s New? – Sam Thursfield