Kammala wannan jumla: Bayan wani ɗan gajeren lokaci a matsayin mai ba da shawara na farko a cikin tsohuwar mai hallaka, daga watan Satumba na 1939 zuwa Afrilu na 1940, an nada Gretton a cikin ɗan gajeren lokaci a matsayin mai ba da shawara na farko a cikin babban mai hallaka zamani kuma ya ga aikin a
Yaƙin Narvik na biyu yayin Yaƙin 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.