I don't recommend freezing au gratin potatoes, as they are super dairy-rich. Dairy products, like milk, cheese, or cream, tend to separate during the freezing and thawing process. So, it's best to prepare the recipe considering the number of people who will consume it. Also, when it comes to storing your leftover potatoes au gratin, use an airtight container and keep it in the refrigerator for up to 4 days.