You can't get Hyper Voice on Gardevoir in X&Y without transfering. Hyper voice is a move tutor move in Black 2 & White 2 and in Omega Ruby & Alpha Sapphire. So the easiest way would be to transfer your Gardevoir to one of these games and then teach it the move. If you don't have any of those games, I'm sorry but the best thing you can do then is to look for trades.
///Jofly
