The pilot reported that he recently purchased the accident airplane to transition from large airplanes back to small airplanes. He conducted a normal, uneventful flight. When he approached the airport to land, he went through the “GUMP” checklist twice. However, when the airplane touched down onto the runway the landing gear was retracted which resulted in substantial damage to the underlying structure of the lower fuselage. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation During a postaccident telephone conversation, the pilot mentioned that he accidentally merged the “GUMPS” check with a checklist he commonly used for large airplanes. He further reported that the airplane was not equipped with a landing gear horn, therefore, he was not alerted of the unsafe landing gear condition. He further stated that a landing gear warning horn will be installed on the airplane during the repair process.