There are two ways to open liftgate on Elantra GT, one from the key fob and the other by pressing the button on liftgate.
To open the liftgate, press and hold the liftgate icon on the key fob to unlock the liftgate. It will not pop open. Then press the button on the liftgate handle to open the liftgate.
If your car has a smart key, have the smart key in your possession and just press the liftgate handle button and open the liftgate.