I can second that Univeral is generally great about allergies. That said, in a theme park, it is generally always prudent to consider carrying a few snacks with you. I should clarify, I've been to Universal with multiple people with food allergies, but when it comes to wheat, my companion only had to avoid wheat. If you absolutely need to avoid all risk of cross contamination, some options I list might not work. Generally, Universal wants folks to tell staff at each place what their needs are. Do look in the DISabilites section of there boards.
Also important, there is a Whole Foods located just off Universal property. While not cheap, WF carries just about every allergen friendly product on the planet. It is a quick stop if you want to pick up some treats. I think you can also now order delivery, though I haven't done so while at Universal. We usually have a car, so it is a short stop on the way.
Within the parks, Mythos is very good. In the past, they usually offered a delicious risotto, though they changed the exact recipe seasonally, so no promise that month's risotto would be GF. Mythos will also bring warm GF rolls, and offers GF buns for burgers/sandwiches. Staff there has been very responsive.
Property-wide, Universal also offers a number of prepackaged items. If open, the food court of Aventura had a bunch of packaged items that were certified GF. The adjective that came to mind when we visited Aventura was that everything there is very...millennial, including the food. One of our favorites is the choose-your-own Asian soup/stir fry bar. Guests get to choose what veggies, protein, starch they want and then stir-fry or soup. Soup is a bit spicy.; rice is an option, also some sushi.
In the parks, we often go to the HP quick serve places. In 3 Broomsticks the rotisserie chicken salad is good; I think you just have to watch which dressing you choose. They have Ken's prepackaged, so check the label. The salad is not on the electronic menu, it is only on the second menu they have at the registers. Another option is Rotisserie chicken platter with corn +potato. At Leaky Cauldron - I think- the bangers (sausages) in the Bangers and Mash might NOT be GF, but we were able to order side dishes that are GF. The minted peas and roasted tomato are super. I think we were able to get like a side salad or something as an option. They also have roasted root veggies that are very good. Butter beer is safe, but we prefer the fizzy orange juice at the Hopping Pot. Still sweet, but not quite as overpoweringly sweet. They dip the edge of cup in sugar, but you can request to skip it.
We also liked the kebabs at Doc Segrue's. They aren't huge, but tasty, and they come with rice. I also recall getting a cup of grapes or maybe fruit salad there. In the HP areas, you can also buy whole fruits and bags of grapes- or at least in the before times.
Sapphire Falls' Dutch Trading Company has some prepackaged items. In prepackaged, I think they had Cheerios, fruits, chips, trail mix - among other items. Teh special can be tasty, but I'm not sure they can promise no cross-contamination. It seems tome they usually have like a streamed veggie and potato or rice. We were able to eat, but we didn't need 100% no cross-contamination.
At Finnegans, they had a few GF options. I think the boiled dinner was among them, and it was decent. It wasn't the very best corned beef ever, but decent. It comes with like boiled cabbage, carrots, potato.
At Royal Pacific, we like the Orchid Court sushi, but it is only open in the evening past 5pm. The Grab and go breakfast (before times ) had a few options. I think they had some prepacked cereals, whole fruits, and hard boiled eggs.
In Citywalk, I think Breadbox now has GF buns. For sides though, I think you need to steer clear of the fries, or maybe it was the tater tots? One option was GF but I think the other was not. They also have coleslaw. It is a sandwich place though, so I'm not sure about cross-contamination risk. The caprese is really good. A few of the sandwiches can be a bit on the greasy side, so we order without the mayo/cream sauce/aioli. Also nearby is a Moe's Southwestern Grill. In before times, Menchie's was self serve soft-serve yogurt. I think if you avoid the toppings, and they are open, their yogurt would be okay.
Cowfish Sushi was very good. I think they had a number of GF options. Toothsome had some GF options, I think mostly items in the pricier entree section of the menu. We steered clear of the menu desserts (too big). The Confectionary shop has a number of prepackaged sweets. They also have a large selection of macarons.
I think Sal's Market had some GF options beyond this, but I recall we really liked the pre-packed Caprese salad, olives, and fruit salad. I also recall they have a number of packaged items like chips. Otherwise, Portofino is mostly Italian offerings, so maybe not the best hotel for GF.