On June 8, 2017, about 2110 eastern daylight time, a Piper PA-44-180 airplane, N3020M, was substantially damaged during landing rollout at Montgomery County Airpark (GAI), Gaithersburg, Maryland. The flight instructor and two student pilots were not injured. The instructional flight was conducted under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no flight plan was filed for the local flight.According to the flight instructor, he was returning to home base after a training flight that consisted of basic maneuvers, as well as two takeoffs and landings to a full stop. During the final approach the flight instructor configured the airplane for landing and briefed the student pilot. The touchdown was uneventful and during the landing roll the airplane began veering to the right of centerline. The airplane came to a full stop and the instructor performed an engine shutdown before exiting the airplane. On inspection of the airplane the flight instructor noted that the right main landing gear was collapsed and bent backwards towards the aft of the airplane. Examination of the airplane by a Federal Aviation Administration inspector revealed that the aft right wing spar web was damaged during the landing sequence. Further examination of the landing gear revealed that the right aft main landing gear trunnion bracket failed. A visual examination of the fracture surfaces by the NTSB Materials laboratory revealed that there were no signs of fatigued failure within the trunnion, and the bracket failed during an overstress event. A review of the logbooks did not reveal any recent maintenance to the landing gear assembly.