When gloss medium dries, if you find that you don't like the sheen, you can buff it down a bit with #0000 grade (very fine) steel wool. Be certain that the gloss medium is dry, otherwise fibers of the steel wool will get stuck in it. An alternative to steel wool is a Scotch Brite scouring pad which won’t leave debris.
A varnish coat is typically the final coat on a painting, and determines the sheen of your painting when finished. See “What is a Varnish?”