How do you do that prep? There are a lot of websites dedicated to sharing gluten free resources, so start with a simple google of "gluten free + CITY NAME" and see what comes up. Whether or not you find info that way, though, the easiest thing to do is locate a nearby grocery store to your hotel. Grocery stores in Europe often have dedicated gluten free sections, and you can get plenty of snacks to supplement--or, make it deliberate and memorable! Buy a bottle of local wine, some delicious spreads, a bunch of grapes, and some gluten-free crackers, and have a charming little picnic in a city park.
What about restaurant dining? While, at least in big cities, you can find all-gluten-free restaurants, you may also find options at more general places. Trust me when I say that many, many restaurants now have gluten free options available. It's often as easy as pausing in your walking and searching "gluten free near me" on your phone, then scrolling through reviews and choosing something nearby. If you're in a smaller town, you may need to spend a bit more time on your advance research, but I'm confident you'll find something--it might be, instead of something like gluten-free focaccia, an authentic dish that simply avoids gluten to begin with (like a perfect, creamy risotto).