Preheat oven to 350 F. In a large saucepan over medium heat, melt butter. Add in garlic and cook until fragrant. Add in flour and cook for a minute to brown. Pour in milk and add shredded pepper jack cheese, salt, and pepper.
Continue to cook over medium heat, while whisking until thickened and smooth. Layer half of the potatoes into a greased baking dish. Pour half of the sauce on top, layer with the remaining potatoes and remaining sauce.