On 24 Dec, there is only one Glacier Express sevice in each direction. Consequently, the train is fully booked quickly. If the Glacier Express website states that there are no seats, there isn’t any other option to book seats. You must have a seat reservation in order to travel on board the Glacier Express, standing travelers are not accepted.
Alternatively, you can still call the Glacier Express reservations team and ask if there are potentially seats not used by reserved groups: +41 81 288 65 65.
There is, however, a feasible alternate: you can travel the same route using regional trains yet involving multiple changes. To get the timetable you simply enter From: Zermatt, To: St. Moritz, Via: Disentis/Mustér
Kindly note, the Glacier Express is not operated by SBB CFF FFS.