Hello SanroccoMark
Welcome to the SBB Community.
The dining car does not offer sufficient space for luggage. You would have to leave the luggage in the adjacent coach. Unless, you can put it under the seat or between the seats.
Keep in mind that the lower deck of the dining car us reserved for passengers in a wheel chair. Deposing the luggage on the platform of this coach is not appreciated, as passengers with reduced mobility must be able to get on and down the train without any hindrance.
Alternatively, there is a Migros store at GVA airport station offering snacks and food; you could get a picknick there and have it at your seat in the another coach.
Kind regards,
Roland
See a picture here:
https://www.railjournal.com/passenger/main-line/sbb-unveils-new-restaurant-cars/