You can take every train you want in the UK with the pass you have.
The overview on the website only gives the most popular -very touristic- options. I have travelled to many, more obscure and lesser known locations in the UK with passes, and almost everything goes :). It’s one of the countries where a pass is a real pleasure to use.
If you need to reserve seats on trains, do so on https://www.gwr.com/your-tickets/seat-reservations. Reservations in the UK are free, although many trains don’t require them! On longer distances it’s recommended though.
The QR code doesn’t work well with ticket gates, but just show it to a member of staff and they’ll let you in or out.
When are you going? For which dates did you look for?
It might be possible that the train times are not yet uploaded in the rail planner app. The schedule changes on december 11th. You can also check on the GWR-website or any other UK train operator to check where you are going, if you’re confused with the rail planner app.