What was found
AFTER SELECTING LANDING GEAR UP, GOT GEAR DISAGREE MSG, C/W QRH LANDING GEAR DOWN 3 GREENS. NLG DID NOT COME UP. RETURNED TO ORIGINATING AIRPORT. ***AIRCRAFT REMOVED FROM SERVICE*** REMOVED AND REPLACED NLG DOOR SELECTOR VALVE IAW AMM 32-22-20. OPS CHECK GOOD. REMOVED PLACARD REMOVED FROM MEL P/N: 750006000, S/N OFF: 1945, S/N ON: 0365T ARW