Preheat oven to 450. Line an 8 inch cast iron skillet with bacon, covering bottom and the sides and leaving a third of each strip hanging over sides. Sprinkle thyme over bacon.
Layer half the potatoes onto bacon and thyme, covering the entire surface. Sprinkle 1/4 cup Gruyere over potatoes; season with salt and pepper. Repeat layering with chicken and 1/4 cup Gruyere. Continue layering with leeks, garlic, remaining potato slices, salt and pepper and remaining Gruyere.
Fold hanging pieces of bacon strips to the center of skillet, covering entire top. Transfer skillet to stove top and cook over medium heat, 10 minutes
Cover skillet with foil, transfer to oven and roast 30 minutes. Remove foil and roast til bacon is crisp and potatoes are fork tender, 30 minutes more. Let meal rest 10 minutes, then drain drippings and loosen edges to serve. Try not to let the bacon pieces overlap in the center because it prevents any covered bacon from crisping.