Generally speaking in my experience you don't need a gasket sealant on the dust cap, if you were careful taking it off it'll work just fine putting it back on, and I always fill the cap with grease so whenever I tap it on it fills the whole void with grease so there's no possible air in the system
Could you create a list of the specific tools you used for this job? Thanks, if you can, or cant, and thanks anyway for posting this informative video.