All but one item (cookies and cream mochi) are gluten free. There are quite a few locations around Hanoi and the staff don’t speak much English but there’s no real mechanism for cross contamination and I (a VERY sensitive celiac) have never had any issues. If you don’t intend to eat your mochi right away, be sure to ask for the frozen ones.
As a general note for tourists/new arrivals, the vast majority of the food in Hanoi is going to be completely safe. Most of the best local places have fairly limited menus (so no cross contamination) and with VERY few exceptions, northern Vietnamese food is completely gluten free. Obviously bánh mì is off the table, and you’ll want to make sure to avoid mỳ/mì noodles (which are wheat, yellow, and easy to spot). I avoid duck, since it’s often served with soy sauce and you’ll want to make sure that you say no if you’re offered quẩy (a sort of savory donut stick you put in your phở). If you do that, it’s extremely unlikely that you’re going to encounter gluten (and yes, things like bún chả that have meatball-like components will be fine). Vietnam is one of the best places in the world for celiacs and general food safety is extremely good, so don’t miss out on the amazing food here. Happy eating!