It’s just that easy.
A knife spreading mayo on a bun, then on your hamburger patty which was next in line is cross contamination and can bring about a painful intestinal attack.
The best way to avoid this is just not to eat out. If the risk is worth it, please inform your server discreetly but effectively, that you are allergic to gluten and nothing gluten related can touch your food. Some people find this embarrassing, but you shouldn’t, it has become more and more common. We have noticed that just over the last couple of years, we are getting less "funny looks" and more understanding and educated feedback from our servers in restaurants. It has become so common that some restaurants advertise gluten free meal selections, as well as taking every step to avoid cross contamination of your food. Godfather's Pizza now has a "trial" gluten free pizza, available at some but not all of its store locations. The entire staff seems extremely cautious of cross contamination and well trained.
For the most part though, just know that there is risk with eating out, not as much as before, but it is still there.
Cook as much as you can at home. This is the only 100% sure way that you can eat gluten free.
Be careful out there.