Trattoria al Forno has done fabulous meals for us. Topolino's was also marvelous last trip. Citricos and Cali Grill are also favorites of ours.
I need gluten-free, dairy-free and DH is allergic to all sorts of peppers. We're something of a challenge, but TalF comes through deliciously for us every time.
Really, the Signatures generally will do a fabulous job for gluten-free diners. Other TS can be hit and miss, we've found.
If you know what sorts of dishes she may enjoy, you might look over the allergy-free Disney menus. Scroll down to the bottom of each official menu for those. Some just say in parentheses which allergies the dish is safe for, others will list safe dishes by allergy--gluten, dairy, fish/seafood, etc.
Or simply suggest she do that to help decide somewhere you all will enjoy. Many gluten-free dishes will be modifications of those on the regular menu.
When making the ADR, be sure to specify gluten-free as well as alerting your server which person is gluten-free. They'll do as much as they can to provide your guest a safe and memorable meal.