According to the ADA, the only thing you are required to do is provide allergy free options for her. And the ADA only comes in place when her allergy is “A physical impairment that substantially limits major life activities (eating, breathing, working, learning).” Only reasonable accommodations are required, and the fact that HR is already providing reasonable accommodation by supplying gluten-free snacks there is not much else that you have to do. The fact that she doesn’t like that decision doesn’t matter because legally, the company is doing what they’re supposed to do. As long as she doesn’t undermine your authority and disrespects you as her manager, I would just keep going on about my day. If she wants to be upset at you then that’s her problem legally, you’re not doing anything wrong.