In his youth Garstin worked as a lumberjack in Canada and as a ranger in Africa.
During the First World War Garstin joined The King's Overseas Dominions Regiment. He received a battlefield commission in 1915.
Disappeared after a dingy he was rowing capsized in Salcombe Harbour. Because his body was never found and he was known to be a strong swimmer, some felt he may have faked his own death.