Hello winwinclaire
Firstly, hi and welcome to the SBB Community.
For the Gotthard Panorama Express you need a 1st class Saver Day Pass or an equivalent 1st class ticket.
The luggage service is included, please see the information on the GOPEX website:
https://www.gotthard-panorama-express.ch/en/stories/luggage-service
When going to Arth-Goldau, you will skip the boat ride, which is unique and scenic.
Kind regards,
Roland