Having won the second running of the Eventing Grand Prix in 1999 on Just So I, Gary then had back-to-back wins in 2008 and 2009 on Peter Street.
He finished second last year, behind Shane Breen, and in this year's reprisal of the popular event riders versus showjumpers showdown, Gary and last to go Peter Street put in a blistering clear to win for an incredible fourth time.
Gary is now going to retire Peter Street from top flight competition. "It's a lovely way to finish his career - he's 17 now and over the years he's been brilliant," said Gary. "You need a very special horse to jump this course, and a horse like that takes a bit of finding."
Germany's Marcus Ehning won the Bunn Leisure Trophy earlier, claiming the £6,500 first prize on board Noltes Kuchengirl.
"She has a lot of experience and I've been riding her for a long time, so I know her well and what she's capable of," Marcus said of the mare.
Riders had an early start this morning, with 79 starters in the Royal International Vase and action kicking off at 8am. Guy Williams put in an unbeatable time on board Djakarta. "She's a young horse but she's really competitive. She'll spend the rest of the week in the smaller classes but you have to give them a taste of the big arena at some stage - and she just blew me away."
In the showing classes , the winner of the Leeman Supreme Hunter Championship was Loraine Homer and the six-year-old Major Moylaw. Loraine, who is the daughter of showman David Tatlow, claimed the tricolor after heading the heavyweight hunters with Mr and Mrs Alan Hall's impressive 17hh gelding.
"He has a wonderful attitude, and just wants to be a show horse," said Loraine. "He walks in the ring and feels like he owns it. I've been lucky enough to win a lot of major classes but today's win was best reception I've ever, he's such a popular horse."