Prior to commencing her distinguished career as one of Australia's most accomplished equine trainers, a young Gai Waterhouse diligently pursued her higher education at the esteemed University of New South Wales. Upon completing her academic endeavours, she embarked on an acting career, gracing both Australian and English television screens with her presence, but found London was not for her. Although a young Gai Smith did appear in a 1978 episode of Doctor Who alongside the Dr, Tom Baker. Nowadays renowned as the preeminent figure in Australian racing, Gai is the esteemed daughter of the legendary Sydney trainer, Tommy J. Smith. The story of Tulloch Lodge began in 1941 when T. J Smith, Gai’s father, received his training license. During the next fifty years, Tulloch Lodge produced some of the best horses ever to run in Australia, including the legendary Kingston Town and Tulloch. TJ became the most successful trainer of Group one winners in Australia, training a record 282 winners at the highest level, winning two Melbourne Cups and receiving 33 consecutive trainer’s premierships. Gai diligently honed her skills under her father's expert tutelage for an impressive 15 years from 1978 at Randwick before meritoriously obtaining an Australian Jockey Club license in 1992, but not after a long legal battle. She applied to the Australian Jockey Club on August 8, 1989, but was discriminated against because her husband, bookmaker Robbie Waterhouse, was banned from racecourses for his involvement in a scandal where the poorly performed horse Fine Cotton was substituted with a better-performing horse disguised with paint and hair dye. Waterhouse took her fight all the way to the High Court and was given approval to train 878 days later.

Her early training journey commenced with the triumphant victory of her first winner, Gifted Poet, in March of that year. Later, in October, she achieved a maiden Group one triumph with Te Akau Nick in the prestigious Metropolitan Handicap. Waterhouse assumed control of her father's Tulloch Lodge Stables in 1994 when he became unwell and promptly achieved success. Notably, the talented colt, Nothin' Leica Dane secured victory in the Victorian Derby and came a close second in the Melbourne Cup. During the 1996-97 season, she remarkably trained ten winners and claimed her inaugural Sydney Premiership. Subsequently, in 2001, she accomplished another extraordinary feat by training the 1st, 2nd and 3rd place finishers in the Golden Slipper, leading to her first of three consecutive Sydney premierships. Furthermore, in 2002/03, she notably equaled her father's impressive Sydney training record of 156 wins. In 2004/05, Waterhouse achieved another outstanding milestone by training eleven Group one winners and securing a fifth Sydney Premiership. The evidence of Waterhouse's exceptional acumen in managing the family enterprise is reflected in her outstanding achievements. Her remarkable success demonstrates her innate talent for identifying champion horses and her proficiency in operating a business that hinges on the often unpredictable dynamics between horse and rider.

Throughout her career, Gai Waterhouse has interacted with a diverse group of prominent individuals, including business magnates as well as esteemed figures like Queen Elizabeth II. Her extensive network and expertise have solidified her position as a leading authority in the racing industry, while her pioneering spirit has paved the way for women to excel in a field traditionally dominated by men. Although a famous stoush with owner John Singleton over his mare More Joyous' poor showing in the All Aged Stakes blew up into a full-blown drama on live TV when Singleton sacked the trainer and removed all his horses from the stable. The iconic billionaire owner claimed her son, Tom Waterhouse was tipping people not to back the horse and it couldn't win, proved by Gai announcing to stewards that the horse had heat in her neck during the week and failed to inform them.

To date, she has trained 158 group one winners, including seven Golden Slippers. She has also trained one of the late Queen’s horses, Carlton House. Waterhouse and her co-trainer, Adrian Bott, have won 4635 races in the past decade, with the pair having won more than $326 million in prizemoney for their owners. As a team they are renowned for bringing through well-bred young horses that perform exceptionally at the highest level when first starting their racing careers. A critical concern for Waterhouse, is succession planning. Despite following in her father's footsteps, she has yet to identify a successor from the next generation to assume the leadership role. Consequently, it appears that the renowned trainer may need to broaden her search to find the next iconic female trainer.

Notable victories for Gai as a lone trainer include: Pierro, Golden Slipper 2012, More Joyous, Doncaster 2012, Dance Hero, Golden Slipper 2004, Vancouver, Golden Slipper 2015, Ha Ha, Golden Slipper 2001, Sebring, Golden Slipper 2008, Bentley Biscuit, T.J. Smith 2007, Fiorente, Melbourne Cup 2013, Grand Armee, Mackinnon Stakes 2004.
Training in partnership with Adrian Bott bought further success, including: Global Glamour, Flight Stakes 2016, Global Glamour, Thousand Guineas 2016, Pinot, Victorian Oaks 2017, English, Doomben 10,000 2018, Prompt Response, Tatt’s Tiara 2018, Con Te Partiro, Coolmore Classic 2020, Farnan, Golden Slipper 2020, Shout The Bar, Vinery Stakes 2020, Con Te Partiro, Coolmore Legacy 2020, Shout The Bar, Empire Rose Stakes, and multiple Gr.1 winner, the fantastic horse, Aligator Blood