Shortly after departure, the 15,000-hour commercial pilot reported that he was unable to retract or extend the main landing gear; however, the nose landing gear continued to function normally. The pilot returned to the airport and landed on a grassy area adjacent to the 5,035-foot-lon, by 40-foot-wide asphalt runway 08, with the main landing gear partially extended. The single-engine airplane slid across the grass before it came to rest upright and tilted to the left. The pilot egressed the airplane normally and proceeded to lock the gear in the down position after the airplane was towed to the hangar. The pilot reported that the horizontal stabilizer sustained minor damage, so he elected to depart the airport and flew to another airport without incident. Examination of several photos taken by witnesses revealed that the left horizontal stabilizer had sustained structural damage and the outboard section was bent upwards. The reason for the main landing gear to fully extend could not be determined. The weather at the time of the mishap was reported as clear skies, with 20 miles visibility, and calm winds.