Rubuta ci gaba ga wannan sakin layi - Bayan wani ɗan gajeren lokaci a matsayin babban jami'in a cikin tsohuwar jirgin ruwa, daga watan Satumbar 1939 zuwa Afrilu 1940, Gretton an nada shi a cikin ɗan gajeren lokaci a matsayin babban jami'in a cikin babban jirgin ruwa na zamani kuma ya ga aikin a yakin na biyu na Narvik
a lokacin yakin Norway. An ba shi umurnin tsohon mai hallaka a 1941 kuma ya yi aiki a Arewacin Atlantic. An ci gaba da zama kwamandan kwamandan a ranar 1 ga Yuni, 1942, an ba shi umurnin sabon mai hallaka kuma ya koma Bahar Rum. Ya shiga cikin Operation Pedestal, aikin jigilar Malta a watan Agusta 1942, kuma ya nutsar da jirgin ruwa na Italiya "Dagabur" ta hanyar ramming. An ci gaba da zama kwamandan a ranar 31 ga Disamba, 1942, an ba shi umurnin mai hallaka, a matsayin Babban Jami'in Escort zuwa Escort Group B7, wanda ke zaune a Derry.