The flight instructor reported that, during a stage check for the commercial pilot training course, the private pilot student completed the "G.U.M.P. [gas, undercarriage, mixture, propeller] check" on downwind in the traffic pattern. He added that, before the turn to the base leg, "everything was normal, and the gear was selected down by the student." He added that he observed three green landing gear extended indication lights illuminated. He further added that, after a normal landing touchdown, when the airplane slowed to 40 knots in the ground roll, the right main landing gear collapsed, and the airplane veered off the runway to the right. He reported that he did not visually check to see if the right main landing gear were extended. The private pilot reported that, "on downwind we followed the G.U.M.P. checklist and verified that the landing gear were down. My instructor checked the right [main landing gear] and I checked the left [main landing gear]." He added that, on base, he "checked the landing lights with green [lights]." He further added that, after a normal touchdown, the right main landing gear collapsed, and the airplane veered off the runway to the right. The right elevator sustained substantial damage. The Federal Aviation Administration Aviation Safety Inspector performed a functional test of the airplane's landing gear system 1 day after the accident. The inspector observed the landing gear retracting, extending, and locking down into place "several times." He added that, during two gear extension cycles, he "simulated an air load on the right main landing gear by pulling back on it as it extended; the gear extended and locked down properly without discrepancies." According to a commercial pilot witness, while he was driving a car along an airport perimeter road, he had a "head-on-view of the aircraft landing." He added that he observed a "red and white C172RG" airplane on final approach that "appeared to not have the gear down." He added that he stopped his car and continued to watch the airplane, and as it passed off to his right, he observed the "front wheel" down and both main landing gear "hanging." He subsequently observed the airplane touch down on the left main landing gear first and then skid off the runway to the right. It is likely that the landing gear selector was moved to the "down" position on short final approach, which did not allow sufficient time for the right main landing gear to fully extend and lock into place.