Gabriel Marie "Gai" Waterhouse AO (née Smith; born 2 September 1954) is an Australian horse trainer and businesswoman.

The daughter of Tommy J. Smith, a leading trainer of Thoroughbred racehorses, Waterhouse was born and raised in Sydney.

After graduating from the University of New South Wales, she worked as an actor for a time, appearing in both Australian and English television series.

In 1994, after her father became ill, she took over his Tulloch Lodge stable, and she has since trained 145 G1 winners and won seven Sydney trainers' premierships.

[2] Waterhouse is the daughter of Valerie and Tommy J. Smith MBE, a leading trainer of thoroughbred racehorses, who was based in Sydney.

[1] Waterhouse first achieved fame in Australia when Nothin' Leica Dane came into the Melbourne Cup in 1995 after winning the Victoria Derby three days earlier.

In the 2012 Caulfield Guineas Waterhouse's hot favourite Pierro, undefeated in his first eight races, was beaten by All Too Hard in a major upset.

[citation needed] Waterhouse was inducted into the Australian Racing's Hall of Fame in November 2007, following in the footsteps of her late father, T. J.