Gretton joined the Royal Navy as a cadet at the Royal Naval College, Dartmouth, Dartmouth. He served in the aircraft carrier "Courageous" before seeing action in the cruiser HMS "Durban" during the Abyssinia crisis and the Spanish Civil War. He led a landing party in Haifa during the Arab rebellion in Palestine. He attended an anti-submarine course at Portland and, on the outbreak of the Second World War, was assigned to the destroyer HMS "Vega" as first lieutenant.