With the roundtrip ticket you are able to take the gondola both ways, the same day.
When you purchase a roundtrip ticket, you can go straight to the ticket gate.
Use the QR-code that you recevied both as an e-mail and as a message to get through the ticket gate. This QR-code is only valid once, eiter at the lower station or at the top station.
This ticket is valid for the date you choose.