Potato gratin and potato Dauphinoise are very similar, both having cream (cream, not milk!) and cheese. Basically a Dauphinoise has more cream for a richer taste, while gratins include more cheese to create a crisper crust.
(note that any dish that is topped with cheese or breadcrumbs and a browned crust is a "gratin").
You got neither, just "potatoes cooked in milk". Seems the way to do it is cook them in milk on a stove top gently first then move to the oven for a final browning.
Still. Full marks for trying.