Will the Gardena card give us access to the Seceda cable car, or do we need to buy those tickets separately?
A quick check on their website would have shown you that it does.
It can be a bit confusing, but the Gardena Card does include Seceda. On the website for Seceda, there is a link at the top of the page for reservations if you have the card, and you’ll need to enter your card number. You can reserve three days in advance. (Not to be confused with the Gardena Mobile Card for access to public transportation offered by most hotels in the area.)