A pilot reported the left gull-wing door came open on initial climb and they returned to the airport.

Narrative

During the runup/before takeoff portion of the flight; I checked to ensure both left and right side doors were secured. This was done by physically pushing on the doors and visually ascertaining the door locking handle was in the locked (forward) position. Both doors were verified secure and locked.On takeoff on Runway XX and initial climb at about 650 feet MSL; I noted sudden airflow into the cabin associated with wind noise. On inspection; I observed the leading edge of the left gull-wing door was open about one inch. Immediately on touching the door handle; the door opened fully; flipping completely over the top of the fuselage.The airplane yawed slightly; I believe to the left; which was easily corrected with rudder input. There was no substantial roll motion. I immediately reduced power; but the airplane was unable to maintain altitude. I resumed full power; notified ATC of my intention to land immediately; and attempted to climb. I was able to eventually reach about 800-900 feet MSL on downwind. I was cleared for a short approach which I executed and landed successfully on Runway XX. I maintained full power until established on short final. On post landing inspection; I noted the door latch remained intact. The door remained attached to the aircraft. There was damage to the door; essentially a horizontal crease where the hinge exceeded the normal opening limits. There was also damage to an antenna on the top of the airplane.A&P evaluation pending.I was not fatigued or rushed. Immediately prior to this flight; I had just completed three circuits in the traffic pattern for passenger currency. These were completed without incident. I did have a first-time passenger aboard.