Place leeks, broth, 1/2 teaspoon salt and pepper in a 6-quart slow cooker. Cover and cook on low for 4 hours until leeks are very soft.
During the final 80 minutes, heat the oven to 425°F. Prick potatoes with a fork, rub with 2 teaspoons oil and roast on a rimmed sheet pan for 60 to 70 minutes until soft throughout, then cool just enough to handle.
Split potatoes and scoop their flesh into the hot leek broth. Cut the empty skins into short strips and toss with remaining oil and salt on the sheet pan. Arrange bacon on a separate foil-lined section of that pan.
Roast skins and bacon at 425°F for 12 to 18 minutes, removing skins as soon as crisp and cooking bacon until browned and crisp. Drain and crumble the bacon.
Blend the potato flesh and leek broth with an immersion blender only until smooth, without prolonged blending. Stir in milk, cover and heat on high for 15 to 20 minutes until steaming.
Switch the cooker off and stir in Gouda and sour cream until smooth. Divide among 6 bowls and top with bacon, scallions and crisp potato-skin strips immediately before serving.