How do I make a deck glossy?
The other answers not withstanding, please keep in mind that a glossy finish will probably be more slippery when it is wet (maybe even when it is still dry). This  may be why you are having a hard time finding a glossy deck finish. I think you will find that surfaces that are great for traction and safety (rougher, sandpaper like) will be the exact opposite of what you need for a great gloss finish and vise-versa.