The pair, who previously worked together at Marcus at The Berkeley, led the opening of the restaurant with rooms last summer.
A representative told BigHospitality they both left the restaurant in May for personal reasons.
McDermott previously cooked at the Michelin-starred Yorke Arms and the three-AA-rosette Orangery Restaurant at Rockliffe Hall.
He has created a new menu working alongside the head gardener at the nearby Netherby Hall mansion house - which is also run by the Inn's owners Gerald and Margo Smith - where the restaurant sources the majority of its fruit, vegetables and herbs.
Gerald told In Cumbria earlier this year that they were hoping to make the restaurant a “one or two star” Michelin experience.
“Gary brings with him a wealth of experience and we are very pleased to welcome him to Pentonbridge Inn,” say the Smith's.
“We know he will continue to drive our restaurant forward as one of the best places to eat in Cumbria.”