A B747-400 FLT CREW DEPARTING ON AN OVERWATER FLT EXPERIENCED AN EICAS MESSAGE 'GEAR DOOR'. THEY COMMUNICATED WITH DISPATCH AND MAINT AND DECIDED TO RETURN TO THEIR DEP ARPT.

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AFTER GEAR RETRACTION; AND IN THE CLEAN-UP PHASE; AN EICAS CAME UP 'GEAR DOOR.' THE RELIEF PLT GOT THE FLT MANUAL PROC OUT AND WE FOLLOWED IT. THE EICAS REAPPEARED AFTER THE PROC WAS COMPLETED. WE THEN CONTACTED DISPATCH WHO IN TURN CONTACTED MAINT CTL. AFTER SOME DISCUSSION; THEY WANTED US TO CYCLE THE GEAR; WHICH WE DID. THE EICAS REAPPEARED. WE DISCUSSED IN GREAT DETAIL THE OPTIONS OF DUMPING FUEL AND LNDG OVERWT. DISPATCH AND MAINT DETERMINED THAT AN OVERWT LNDG WOULD BE ACCEPTABLE AND ALL PERFORMANCE FACTORS WERE CALCULATED AND CONSIDERED. AFTER CONSIDERATION BY ALL FLT CREW MEMBERS; THE DECISION WAS MADE TO LAND OVERWT. ALL PREPARATIONS WERE MADE AND WE GOT VECTORS BACK TO ARPT. I ELECTED TO PERFORM THE LNDG AND ASSUMED THE PF DUTIES. AN APCH AND OVERWT LNDG WAS MADE UNDER THE CAPT'S EMER AUTH. A SMOOTH TOUCHDOWN WAS MADE IN THE LNDG ZONE AND MAX REVERSE THRUST WAS APPLIED WITH MINIMAL BRAKING. THE ACFT WAS TURNED OFF THE RWY AND PARKED AT THE GATE. BRAKE TEMPS NEVER EXCEEDED 3. THE LNDG WT WAS 737.5 AND APCH SPD WAS 175 + 5 KTS FOR WIND CORRECTION. FLAPS 25 DEGS WAS SELECTED DUE TO FLAP SPD LIMITS.