The former Great British Menu chef will leave the Sussex hotel in April and has yet to reveal his future plans.
Gillan joined the Camellia restaurant at the Exclusive Hotels-owned South Lodge in 2006 and launched The Pass in 2008.
The restaurant has held a Michelin star since 2011 along with four AA Rosettes.
Gillan said his decision to leave was not ‘an easy one’ but that the ‘time was right’ for him to move on.
“South Lodge and the Exclusive team have given me an incredible platform at The Pass to showcase my skills and creativity and I feel enormously privileged and proud to have achieved so much throughout my ten years,” he said.
“I shall be very sad to say goodbye but the time is now right for me to move on and I am looking forward to my next venture.”
Danny Pecorelli, managing director of Exclusive Hotels and Venues, said the group was ‘really proud’ of the departing chef.
“Matt’s experimental originality and playful showmanship has secured The Pass’s reputation as a vibrant destination restaurant, so I’ve no doubt of his future success and wish him all the very best,” said Pecorelli.
Exclusive Hotels has a track record of attracting award-winning chefs to its country house venues, including Matt Worswick at Pennyhill Park, Olly Rouse at Lainston House, Rob Potter at The Manor House and former head chef of The Camellia at South Lodge Steven Edwards.
The group runs six hotels and two golf clubs across the UK.