When he was about 20 years old, Gilbert was living in New South Wales near Forbes. He became a member of a gang which was led by Frank Gardiner. Another member of the gang was Ben Hall. With Gardiner and Hall, Gilbert was involved with the biggest gold robbery in Australia's history: in 1862 the gang robbed a coach carrying gold at Eugowra. After the robbery, Gardiner went to Queensland to hide from the police. John Gilbert went to New Zealand. When he came back in May 1863, he started a new gang with Ben Hall.