Trim fat from ham slices, leaving 1/4-inch fat around edges. Score fat at 1-inch intervals to prevent slices from curling.
Heat a large skillet over low heat. Place first ham slice in skillet, fat side of ham near edge of skillet. Place second slice on top of first, with fat side of ham near opposite edge of skillet. Stack remaining ham slices, overlapping each slice so that fat side is near edge of skillet. Cover and cook over low heat 10 minutes. Remove ham from skillet. Turn over each slice, and return slices to skillet, repeating stacking procedure. Cover and continue cooking an additional 10 minutes. Transfer ham slices to a warm serving platter, and set aside.
Drain pan drippings, reserving 1/4 cup in skillet. Add flour to drippings, stirring until smooth. Cook over low heat 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Serve gravy with ham.