, a Piper Navajo, was on approach at Winnipeg when the pilot reported that he had an unsafe gear indication. He carried out two fly-bys of the Tower and it appeared the gear was fully down, but the gear doors appeared abnormal. The pilot proceeded south of the airport to work on the problem, but could not rectify the unsafe gear indictaion. The a/c subsequently landed safely on Runway 18 with ERS on standby. The pilot shut down on the runway to conduct an inspection and about 10 minutes later, started up and taxied to the apron without problem. OPI - Maintenance and Manufacturing UPDATE M. & M. reported that after landing, a maintenance crew inspected the aircraft and found that the L/H main landing gear downlock hook had not fully engaged. They then engaged the hook fully and the aircraft started up and taxied to the hangar. The hook assembly was found to be full of mud and old grease. Once cleaned, the system was tested serviceable by means of a gear swing. The operator agreed to submit an SDR.