what is the most easiest way to return from dragon gate to Kunming Airport.

Well, first of all, you need to arrive at the airport at least two hours in advance before the departure. Thus I'm afraid that the sightseeing time is a little bit tight. You can also take the cable car to come down.
About the transportation from the Dragon Gate to the airport, take bus no.94 from Haigeng Cable Car to Dianchi Road and bus no.A9 to Shuangqiaocun. Then take subway line 1 to Chunrongjie Station, Exit B. Walk northeast around 450 yards to take airport shuttle bus line 4 to the terminal building.