I would like to make this gluten free, is there a good substitute for the flour used in this recipe

Interesting recipe. I've never used flour in a pot roast recipe, so I'd think you could just omit it. OTOH, the crust sounds particularly appealing. I think I will try it with potato starch. That's usually my go-to sub.

I bet arrowroot would work well.

In other recipes like this, I've seen people use potato starch or rice flour, but haven't tried it here. You could also just omit the flour and it would still be delicious -- it just wouldn't have quite as much of a crust.