You could try putting a coat of spar uerathane over it, but you could be opening a can of worms. It could peel/flake/crack in a short time, then be really hard to re-finish. I tried it myself years ago on a commercial project cause they wanted a gritty surface because parts of the deck were sloped to match with concrete work, therefore slippery so I added grit into the eurathane. It looked/worked great, but because of the large amount of foot traffic it was wore off in 3-4 months.

Another down side is the stuff is costly, doesn't go as far as most stains & is harder to put on cause it's thick.

The stain/sealer I used was not the same as what you used, it was superdeck which is an easy one coat application that looks good, but just doesn't hold up well here. The GC supplied the stain so that's what I had to use, would not have been my choice which is TWP.

Since you said you were happy with the stain you have I'd advise leave it alone.

I second Deckman's answer. There are no glossy deck sealers. There are exterior polyurathanes out there, but they are not meant for deck surfaces.

I've used them on some outdoor furniture and it truly looks great, but I have to sand it down and redoo it every year.

It is possible to have a low gloss finish on an outside project by building coats with twp 100 series that will last 3 years or so.

It requires Work/time. J.