[wvguy]

ok .. clear gloss topcoat. Not something i would advise since it can make your deck very slick with water. I am sure it would be a oil based product topcoat. Someone may have put say a interior/exterior varnish over the deck stain.
Two options. First you could sand, rent a floor sander and sand it all down. or use a stain stripper. Depending on how large the deck is would determine the method. The floor sander would be quicker on larger jobs but little more work.
After removing that top coat. and yes you will have to remove the top gloss coat or nothing will penetrate to the wood fibers.
See what it looks like after sanding or stripping then you can determine if you need a wood toner or a semi-transparent stain. Remember its always best to brush stain not spray.
Hope this helps.