Archbishop Michael Kelly founded St Gabriel's. He knew of the Cabra Deaf Boys School in Ireland and the good work being done there by the Irish Christian Brothers in caring and educating deaf boys of Ireland and wanted the same for the deaf boys of Australia. 

On a visit to Dublin in 1920, he requested the Superior General, Brother P J Hennesey, to open one such deaf school in Sydney and obtained from him the promise that Brothers would be sent from Cabra to the school in Sydney.

On his return to Sydney, he had the Christian Brothers commit to find a suitable property in Sydney " to establish a school for the Catholic deaf and dumb boys of Australia." After many weeks searching, a property was selected at Castle Hill. It consisted of "a fine old residence, a cottage nearer the road and 35 acres of good land."

Five months after the land was secured, as promised, two Christian Brothers who had taught for some years at Cabra set sail from London and arrived in Sydney on June 10, 1922. On 10 July the first sod was turned in the erection of the school at St Gabriel's.

St Gabriel's was founded on September 10, 1922 when the foundation stone for the first block of buildings was blessed by Archbishop Kelly and the school was formally opened on May 6, 1923.

Perhaps one of the most significant changes in St Gabriel's history occurred in 1973 when it changed form a residential day school for deaf boys only to a co-educational day school with the first girls attending.

1994 saw another historical change when for the first time since its beginning, St Gabriel's was without a Christian Brother as Principal. Elizabeth Kerr became the first layperson and the first woman Principal. In the intervening years the school has had eight Principals and is currently lead by Jon Franzin who is also the Principal of St Edmunds at Wahroonga.

Over the years the school's enrolment patterns have changed, and in response broadening to include students who have sensory impairments other than hearing, as well as students with other special needs. The school's mission is still centred on the education of students with hearing impairment. The basic philosophy is focused on meeting the individual needs of each student, helping them to develop skills that enable them to take their rightful place in society.