When I made our GE reservation, I didn't make a food reservation. I understand that you can bring your own food on, but do they offer food purchases on the train without reserving it in advance?
In addition to the dining car there is also a snack bar, where you can purchase beverages, candy, baguettes etc to bring back to your seat.
There is a dining cart that comes through (at least there was a couple of years ago). If you are traveling from Brig east to Chur, you must purchase the wine glasses with the tilted stems. You can eat lunch as you are going up to the Overlap pass and your glass of wine will be level. It's a novelty and the wine glasses are a rip-off. Yes, we have our pair of glasses. Also, as I recall, lunch was about 40-50Euro for two and it wasn't that great.