I have stayed at Orisson many times, and have seen them accommodate gluten-free guests for dinner on numerous occasions. Just be sure to tell them what you can't eat when you check in. They changed their standard pilgrim's menu this past year, to a main course of meat and potato stew over macaroni. They serve a delicious vegetarian soup to start, but that may have flour in it. Vegetarians get a nice mixed salad with a hard boiled egg as the main course, so that could be an option.
There is no problem bringing along your own food for lunch, which you can enjoy on the terrace or inside in the dining room (we've done both). And they sell food during the day at the bar, which will include some gluten-free options.
Breakfast is what really may cause you trouble. They serve bread with jam, plus coffee, tea, or hot chocolate. I always bring along yogurt, fruit, and oatmeal, all of which is available at the Carrefour supermarket in SJPP.
And be sure to pick up something for the walk from Orisson to Roncesvalles, as the only thing Orisson offers to take with you are meat and cheese sandwiches on a big brick of bread. There will likely be a food van about two hours after Orisson, where Michele or his son Sebastian sell hard boiled eggs, coffee, fruit, and sheep's cheese from their farm.
Hope that helps!