What were you planning to pack for the rest of your family for that 7 hours?
Since you're staying at a nearby hotel the night before, I'm guessing you're on the 6:15 or 6:35am flight. I tend to "live off the land" when I travel. I usually toss a couple of 2-oz packets of nuts in my carryon and an empty water bottle and call it good unless I know that the airports will be particularly desolate. A Hudson News usually contains a surprising amount of edible, gluten-free food.
There's not much at SEA in concourse B. Starbucks carries Kind bars, and there's a Qdoba. While Qdoba is open 24/7 (and will probably have plenty to time at 5-6am to take extra precautions) - it's not really what I'd want for breakfast food. Picking up a salad with plenty of protein on it and figuring it will keep acceptably until you're hungry would be an option for my DH. Oh, there's a Wendy's that will be open too, if you're feeling Wendy's inclined.
The 6:35 stops in Phoenix (Sky Harbour) in Terminal 4. You don't have a really long layover, but you can probably snag something fast. There's an Einstein Bros, which has gluten free pre-made salads (all the ingredients are right on the packaging as well as on their website) and fruit cups, fruit/yogurt parfaits (I toss the seperately-packed granola). Those are all pre-made in big batches, so cross-contact risks are minimal. They're also easy to snag from the cooler and check out without much waiting.
If you're on the 6:15 flight with no layover, I might augment my bag with some fairly study cookies, but from 6:15-1:15 (ignoring the time changes), I'd tend to eat breakfast (Mmm, black coffee, kind bar and banana), and then snack a little. There's a Chick-fil-a at MCO, and a green salad w/ chicken is worth waiting for, in my opinion.
Note that I travel a lot, so finding edible gluten-free food in airports, gas stations and quickie marts has become second-nature. If you're nervous about finding enough - by all means make yourself comfortable, but it probably isn't strictly necessary.