There's no gasket - just RTV silicone sealant. You'll want to remove the cover anyway to clean off the magnet on the inside. Once the cover is off, jack up one rear wheel, lower it, jack up the other side, and lower it, to drain all the gear lube out of the rear axle. Clean off the diff and the cover real well, put on new sealant, button 'er up and fill to the hole.
Dan Heynen