Gliding takes no energy, but it requires many sensors and a very clever control program. I do no think that anyone has built a drone capable of long-term gliding, yet (i.e. capable to recognize thermal currents and use them for ascending and also capable of gliding a long time without losing much altitude).

Nonetheless, it should possible for a flapping-wing drone to do some short-time gliding between intervals when it uses wing flapping to ascend to the initial height, in order to save some energy.