Universal has a page listing the gluten free options at various restaurants in the park on their website, which is helpful: http://www.universalstudioshollywood.com/gluten-free-dining/
We were only there for lunch (we ate just outside the park at Wolfgang Puck's along City Walk for dinner -- I posted a review for that on this site as well). We opted to dine at the Hollywood Grill, both because it was convenient to where we were when we were ready to eat and because it seemed to have the most options for gluten free diners. It's nothing fancy -- basically your typically theme-park counter-service restaurant. Our gluten free son ordered a burger on a gluten free bun. The woman taking the order, however, clearly wasn't properly trained because she said they didn't have gluten free buns. Fortunately, after asking her to double check with the manager, she was told that they DO have gluten free buns for burgers. At that point, two other employees came over to talk to us and asked if we'd like the bun grilled. We explained that, yes, we'd like it grilled, but it would have to be on a separate area of the grill. There was also some confusion about the fries. They are listed on their website as being gluten free (and apparently they have a dedicated fryer for them), but one of the employees said most gluten free diners opt for a side salad instead. We didn't want to risk it, since there was clearly not a great level of knowledge among some of the employees. I could just see them not being careful and using the oil to fry chicken nuggets, too. Our son said the burger was good, and the bun certainly looked good. He had no negative reaction from the experience. If I had to do it again, however, I'd probably opt to go to Gru's Lab Café, which had gluten free Rotisserie Chicken (as did the Palace Theatre Café). The Jurassic Café also had gluten free Roasted Chicken. Salads and grilled chicken sandwiches are the other options at various other locations in the park. So there are some choices, but not a ton. Ben and Jerry's ice cream and Dip 'n Dots are also in the park. Both offer gluten free varieties.