I know I've seen a gf pot pie made with potato. Mashed potato I think. I don't know though if that would work on top of the pie. Maybe add something to mashed potato.
This one uses corn meal and rice flour.
http://glutenfreemommy.com/easy-chicken-pot-pie/
This one uses potatoes and rice flour
Chicken Pot Pie Filling (Gluten-Free)