Born in Tasmania, Gaby worked as a farmer and then a labourer before enlisting in the Australian Imperial Force in 1916 and volunteering to serve overseas.

[2] Over the course of the next twelve months while in the frontline on the Western Front, he was promoted through the ranks at a rapid speed, reaching sergeant before being selected for officer training in the United Kingdom.

On 8 August 1918, at the start of the Allied Hundred Days Offensive, Gaby was acting as commander of his battalion's 'D' Company, which was committed to an attack around Villers-Bretonneux, France, during the Battle of Amiens.

When the advance was checked by a large German force about 40 yards beyond the wire, Gaby found a gap and approached the strong point under heavy machine-gun and rifle fire.

[1][2] His VC citation from the London Gazette of 30 October 1918 reads: For most conspicuous bravery and dash in attack, when on reaching a wire in front of an enemy trench, strong opposition was encountered.

Lieutenant Gaby found another gap in the wire, and, single handed, approached the strong point while machine guns and rifles were still being fired from it.