Melt 2 tablespoons butter in 12-inch nonstick skillet over medium heat. Add panko and ½ teaspoon salt and cook, stirring often, until golden brown, 3 to 6 minutes. Transfer to bowl; set aside. Wipe skillet clean with paper towels.

Melt remaining 2 tablespoons butter in now-empty skillet over medium heat. Add leeks and ¼ teaspoon salt and cook, covered, until softened, about 6 minutes, stirring halfway through cooking.

Stir in potatoes, broth, wine, and remaining ½ teaspoon salt and spread into even layer. Cover and bring to vigorous simmer over medium-high heat. Reduce heat to medium-low and simmer, covered, until potatoes are fork-tender, 20 to 25 minutes.

Stir in cream and pepper and return to simmer. Continue to cook, uncovered, until spatula leaves trail when dragged through mixture, 4 to 6 minutes longer.

Off heat, stir in Gruyère. Sprinkle evenly with panko mixture. Serve.