Tomatoes are naturally gluten-free unless they are mixed with certain wheat byproducts. It all depends on how they are prepared, because you will find that different restaurants and companies have different ways of doing this.
If you are on a gluten-free diet, it will be important to make certain that the tomatoes you are getting from the grocery store are gluten free as well as in the restaurants you go to.
Naturally this vegetable does not contain any gluten whatsoever, so if you buy them this way from a local grocery store, you will not have to worry about ingesting any.