Slice 3 lb. leeks (5–6 large), white and pale green parts only, halved lengthwise if thick, on a diagonal into 1" pieces, rotating after each cut to create diamond-shaped pieces.

Heat a dry large cast-iron skillet over medium-high. Add 4 slices bacon (about 4 oz.), cut crosswise ½" thick, and cook, stirring often, until browned and crisp, 5–7 minutes. Using a slotted spoon, transfer bacon to a small bowl. Reserve pan with bacon fat.

Add leeks, 6 garlic cloves, finely grated, and a pinch of kosher salt to fat in reserved pan and cook, undisturbed, until leeks are just starting to soften and garlic is fragrant, 1–2 minutes. Add 1 cup water and immediately cover with a lid (a baking sheet also works). Cook until leeks are almost fork-tender, 6–8 minutes. Uncover and add one 17.5-oz. package shelf-stable potato gnocchi, ½ cup heavy cream, and 1½ tsp. Diamond Crystal or 1 tsp. Morton kosher salt; season with freshly ground pepper. Cook, stirring occasionally, until sauce gently clings to gnocchi and gnocchi are tender, 2–3 minutes. If sauce is too thick, stir in more cream 1 Tbsp. at a time until the right consistency. Taste and season with more salt if needed.

Remove pan from heat and top gnocchi with finely grated or shaved Parmesan and reserved bacon. Finely grate zest of 1 lemon over and season with more pepper. Cut lemon into wedges and serve with gnocchi for squeezing over.