Works most of the time. Definitely the best add-on for this purpose at the moment.

The good:
- If a game is on gamepass it will probably tell you
- If it's not sure, it will take a "best guess"

The neutral:
- I haven't found any title displaying "Game doesn't seem to be in Gamepass", it would instead not appear at all, or get a best guess

The bad:
- The "best guess" feature often misidentifies games that are not on gamepass as ones that are/were. it's easy to tell though.