The darned thing is corroded and almost impossible to remove without munging it up with vice grips.
So: Just mash your palm (or thumb if you're ham-fisted like me) up to block the normal flow a couple of times in quick succession. With some aerators, it will flush the gravel out of there by way of the little vents on the top.
(Block the sideways spray with your other hand first or you'll get really wet.)