After a take-off from Frankfurt-Hahn Airport, the aircraft Commander called for ‘gear-up’. During gear retraction, the Flight Crew heard an unusual noise. This was followed by the aircraft rolling to the left, and the illumination of the left-hand landing gear red and green indication lights. The Flight Crew completed the ‘GEAR DISAGREE’ checklist and elected to return to Frankfurt-Hahn. The aircraft landed normally, but as a precautionary measure the Airport Rescue and Firefighting Service attended the aircraft. The aircraft taxied to a parking stand where they were informed by Air Traffic Control (ATC) that a small, metallic piece from the aircraft had been found on the runway. Damage to the upper surface of the left wing was observed by the Crew and passengers. No injuries were reported to the Investigation. There was no fire.