What was found
LANDING GEAR WOULD NOT RETRACT AFTER TAKE-OFF. L/G RETRACTION FAULT ON ECAM. EMERGENCY DECLARED. COMPLIED WITH ECAM AND QRH. EMERGENCY DECLARED. AMM: 32-31-12-04, REMOVED AND REPLACED LANDING GEAR SELECTOR VALVE 3GA, OPS AND LEAK CHECK GOOD IAW ABOVE AMM.