Hi there. Were the gauntlets you acquired Artifice armor? If yes, Artifice armor can't be transmogged. You have to acquire the normal dungeon version of the armor to transmog it. It's also correct that you don't need to dismantle armor in order to transmog. Please note that when an armor is equipped, it will not appear available to transmog because you're already wearing it and have the appearance. Simply equip a different armor piece and it should appear in the transmog menu to unlock.