Switzerland has, probably as the only country in this world, an integrated tariff and ticketing system. Tickets are valid on all means of transport, no matter the operator. Exceptions do apply for mountain railways and cable cars. The STP is fully covering the Golden Pass.
If you want to ride the Golden Pass Panoramic, I suggest a seat reservation. It is well worth the money. It is very popular, and until 20 October 2023 more travellers are expected due to multiple ticket promotions (e.g. ztraveller wit the Coop Day Pass).