Hello Beatricedv
If you have a Night GA Travelcard and you travel between 7pm and 5am (7am on weekends and general public holidays), you do not need to buy a ticket. You can show your Swiss Pass Card or your SBB Mobile App if your SwissPass appears on your phone.
However, if you travel on a different time, you will need to buy a ticket. The seat reservation is not compulsory and it is not included in the travelcard - if you want to have a guaranteed seat, the price of 5.00 francs always applies.
Regards,
Chiara_S