When the First World War broke out on 4 August 1914, Gellibrand offered his services to the commandant of the 6th Military District (Tasmania).
On 20 August 1914, he was appointed to the Australian Imperial Force (AIF) as a captain, and given the post of Deputy Assistant Quartermaster General (DAQMG) on the staff of the 1st Division.
Staff college graduates like Gellibrand were scarce in Australia; only six Australian Army officers had graduated from staff colleges.
There were also four of the British Army's graduates on secondment.
For the 1st Division's General Staff (G) Branch, the division commander, Major General William Bridges, chose two of Gellibrand's Camberley classmates, Lieutenant Colonel Brudenell White from the Australian Army, and Major Duncan Glasfurd from the British Army, and Captain Thomas Blamey, an Australian Army officer who had graduated from the Staff College, Quetta.
The Administration (A) Branch was headed by the AA & QMG, Colonel Victor Sellheim.
Lieutenant Colonel W. G. Patterson was his deputy, the DAA & QMG.
As DAQMG, Gellibrand was responsible for logistics; Major Cecil Foott, an Australian 1913 Camberley graduate, was the Deputy Assistant Adjutant General (DAAG), responsible for personnel matters.
On 23 September 1914 Gellibrand was promoted to major, the usual rank for his post.

The 1st Division headquarters left Port Melbourne on the Orient liner SS Orvieto on 21 October 1914, which reached Alexandria on 3 December.
Sellheim left to command the new Intermediate Base Depot on 12 January 1915.
Patterson stepped up to become AA & QMG, and Gellibrand to become DAA & QMG.
Foott took Gellibrand's place as DAQMG, and Captain Thomas Griffiths became DAAG.
Gellibrand landed at Anzac Cove with the second echelon of 1st Division Headquarters at around 09:00 on 25 April 1915.
As DAA & QMG, he was responsible for supply.
He helped organise the beach parties, rounded up stragglers, and organised the movement of supplies and ammunition forward.
Patterson had a nervous breakdown and was evacuated on 28 April 1915, Gellibrand performed his job as well until Lieutenant Colonel John Forsyth was appointed AA & QMG on 7 May 1915.

Gellibrand was disappointed at not being appointed AA & QMG, but Bridges was unimpressed with Gellibrand's staff work.
He felt that Gellibrand had mishandled the move of the 2nd Infantry Brigade to Cape Helles, where it participated in the First Battle of Krithia.
He also expected Gellibrand to organise a proper officers' mess at Gallipoli and was annoyed at the poor quality of what Gellibrand had scrounged from ships' canteen supplies.
Gellibrand might have even been dismissed by Bridges but fate intervened.
On 1 May Gellibrand was wounded in the ankle by shrapnel.
Then, on 11 May, while laughing at two men whose water bottles had been holed, he received a severe wound in his right shoulder and was evacuated to the hospital ship HMHS Gascon.
While it was still anchored off Anzac Cove, Bridges was wounded on 15 May and brought on board the same ship (and to the same bed, Gellibrand was moved out of the way), where Bridges died on 18 May 1915.

Gellibrand returned to Anzac on 31 May 1915, to find that Forsyth had been given command of the 2nd Infantry Brigade and Foott, who had been his subordinate, had become AA & QMG and was now his superior.
Gellibrand took this in bad grace although Foott was in fact the senior major.
Dissatisfied, Gellibrand put in for a transfer to the 2nd Division, then being formed in Egypt under the command of Major General James Gordon Legge.
He became DAA & QMG of the 2nd Division in Egypt on 21 August 1915.
Because the 2nd Division's AA & QMG, Lieutenant Colonel Thomas Blamey, was forced to remain in Egypt for medical reasons, Gellibrand became acting AA & QMG on 29 August, embarking for Anzac once again on the SS Southland that day.
On 2 September, the ship was torpedoed.
Gellibrand eventually reached Anzac on 6 September.
The August offensive was over, and Gellibrand settled into routine administration.
He was struck down by typhoid again on 11 October a
When the First World War broke out on 4 August 1914, Gellibrand offered his services to the commandant of the 6th Military District (Tasmania). On 20 August 1914, he was appointed to the Australian Imperial Force (AIF) as a captain, and given the post of Deputy Assistant Quartermaster General (DAQMG) on the staff of the 1st Division. Staff college graduates like Gellibrand were scarce in Australia; only six Australian Army officers had graduated from staff colleges. There were also four of the British Army's graduates on secondment. For the 1st Division's General Staff (G) Branch, the division commander, Major General William Bridges, chose two of Gellibrand's Camberley classmates, Lieutenant Colonel Brudenell White from the Australian Army, and Major Duncan Glasfurd from the British Army, and Captain Thomas Blamey, an Australian Army officer who had graduated from the Staff College, Quetta. The Administration (A) Branch was headed by the AA & QMG, Colonel Victor Sellheim. Lieutenant Colonel W. G. Patterson was his deputy, the DAA & QMG. As DAQMG, Gellibrand was responsible for logistics; Major Cecil Foott, an Australian 1913 Camberley graduate, was the Deputy Assistant Adjutant General (DAAG), responsible for personnel matters. On 23 September 1914 Gellibrand was promoted to major, the usual rank for his post.
The 1st Division headquarters left Port Melbourne on the Orient liner SS Orvieto on 21 October 1914, which reached Alexandria on 3 December. Sellheim left to command the new Intermediate Base Depot on 12 January 1915. Patterson stepped up to become AA & QMG, and Gellibrand to become DAA & QMG. Foott took Gellibrand's place as DAQMG, and Captain Thomas Griffiths became DAAG. Gellibrand landed at Anzac Cove with the second echelon of 1st Division Headquarters at around 09:00 on 25 April 1915. As DAA & QMG, he was responsible for supply. He helped organise the beach parties, rounded up stragglers, and organised the movement of supplies and ammunition forward. Patterson had a nervous breakdown and was evacuated on 28 April 1915, Gellibrand performed his job as well until Lieutenant Colonel John Forsyth was appointed AA & QMG on 7 May 1915.
Gellibrand was disappointed at not being appointed AA & QMG, but Bridges was unimpressed with Gellibrand's staff work. He felt that Gellibrand had mishandled the move of the 2nd Infantry Brigade to Cape Helles, where it participated in the First Battle of Krithia. He also expected Gellibrand to organise a proper officers' mess at Gallipoli and was annoyed at the poor quality of what Gellibrand had scrounged from ships' canteen supplies. Gellibrand might have even been dismissed by Bridges but fate intervened. On 1 May Gellibrand was wounded in the ankle by shrapnel. Then, on 11 May, while laughing at two men whose water bottles had been holed, he received a severe wound in his right shoulder and was evacuated to the hospital ship HMHS Gascon. While it was still anchored off Anzac Cove, Bridges was wounded on 15 May and brought on board the same ship (and to the same bed, Gellibrand was moved out of the way), where Bridges died on 18 May 1915.
Gellibrand returned to Anzac on 31 May 1915, to find that Forsyth had been given command of the 2nd Infantry Brigade and Foott, who had been his subordinate, had become AA & QMG and was now his superior. Gellibrand took this in bad grace although Foott was in fact the senior major. Dissatisfied, Gellibrand put in for a transfer to the 2nd Division, then being formed in Egypt under the command of Major General James Gordon Legge. He became DAA & QMG of the 2nd Division in Egypt on 21 August 1915. Because the 2nd Division's AA & QMG, Lieutenant Colonel Thomas Blamey, was forced to remain in Egypt for medical reasons, Gellibrand became acting AA & QMG on 29 August, embarking for Anzac once again on the SS Southland that day. On 2 September, the ship was torpedoed. Gellibrand eventually reached Anzac on 6 September. The August offensive was over, and Gellibrand settled into routine administration. He was struck down by typhoid again on 11 October a
John Gellibrand