The young rider enjoyed arguably his finest moment in the saddle when partnering Brindisi Breeze to victory in the Albert Bartlett Novices' Hurdle at the Festival last March.
Gillies, who was also associated with the Willie Amos-trained Lie Forrit, primarily rode for Perth handler Lucinda Russell after joining her team in 2007.
Gillies' agent Paul Brierley said: "At the present time, all I can say is there has been an accident."
Fellow jockey Zander Voy tweeted: "I can't believe it. You only have 1 best friend and now he's gone. I'll never forget you @campbellgillies, love you pal. xxx."