As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AFTER TAKE OFF AND UPON A LANDING GEAR `UP` SELECTION, A GEAR DISAGREE EICAS MESSAGE APPEARED. THE CREW RAN THE APPROPRIATE QRH, WHICH INCLUDES ACTIVATING THE LANDING GEAR EXTENSION ALTERNATE RELEASE MECHANISM. THE CREW DECLARED AN EMERGENCY AND LANDED AT THE DEPARTURE AIRPORT. SUBSEQUENT TROUBLESHOOTING PER FIM 32-60-07 AND 32-60-09 LEAD TO REPLACEMENT OF THE NLG SELECTOR VALVE AND RETRACTION ACTUATOR.