Whether dextrin is gluten-free or not depends on its original source.
In the United States, dextrin is often made from gluten-free ingredients such as corn or tapioca, making it gluten-free. Sometimes dextrin is made from wheat. Most foods in the United States that contain dextrin made from wheat will label it as so. However dextrin is typically so highly processed that it is generally considered safe for individuals with celiac disease and other gluten-related disorders to consume (as it would contain less than 20 ppm of gluten, making it legal to be labeled gluten-free). However, some people still react to even the tiniest amount of gluten, so be sure to read labels and do what feels right to you.