The jump rider had secured the most significant victory of his career earlier this year at the Cheltenham Festival, riding Brindisi Breeze in the Grade 1 Albert Bartlett Novices’ Hurdle.
He had been in action as recently as Sunday, racing at Hexham, and won 131 races in Britain over the course of a six-year career.
He rode primarily for Perth handler Lucinda Russell, and his 2011/12 campaign was his most successful yet.
He was on holiday on the Greek island of Corfu when he died. According to as yet unconfirmed reports in the press, he lost his life during 'a swimming pool incident'.
Gillies' agent Paul Brierley said: "At the present time, all I can say is there has been an accident."
"Very sad news about Campbell Gillies," said jockey AP McCoy, who led a host of tributes on Twitter, "a very good jockey but an even better bloke.All our thoughts are with his family RIP."
Campbell's brother Finlay plays for rugby side Glasgow Warriors.
The club said in a statement: "Glasgow Warriors are deeply saddened to learn of the sad passing of Campbell Gillies, brother of Warriors hooker Finlay.
"The thoughts of everyone at Glasgow Warriors are with Finlay and his family at this terrible time."