If you are not making this for the Queen, then you can use milk. Because it is an open gratin dish, the excess liquid should bubble away. You may want to add a little extra butter for richness.
Or if you have evaporated milk available, that will be a little closer to the original, although still not the same.
The problem of making it with a bechamel is that the liquid is tied up with the flour and is not as available to cook into the potatoes. That said, many potato au gratin recipes are made with milk and flour, which is very similar to what you are trying to do with this.
No matter what you do, it will not be the same dish, but should be nearly as tasty, if not as rich, as the original.