Gammell, who is gluten intolerant himself, invested in Genius Foods in 2009 after tasting one of the breads made by its founder, Lucinda Bruce-Gardyne.
“At the time, Lucinda was looking for financial backing to commercialise her gluten-free bread,” he said.
“As a long-suffering coeliac, I was over the moon to be able to tuck into tasty bread once again and it was an easy decision to invest in Lucinda’s vision.
“Since then Lucinda and her team have grown Genius into a rapidly-expanding international food group and I am delighted to be able to join the board at this exciting time in the company’s development.”
The company has grown its turnover to £23 million just three years after being set up and last year broke into the United States after signing a deal with American firm Glutino.
Recent contracts have included an order to supply products to French supermarket giant Carrefour for its Spanish stores.
Together, Gammell and his wife, Janice, own a 26 per cent stake in the business, according to documents filed at Companies House.
Janice is stepping down from Genius Foods’ board after serving for three years.
Gammell, who is chairman of Cairn, replaces corporate financier Edward Murray as chair. Murray will stay on the board as a non-executive director alongside Ernie Finch, a former senior group executive at Marks & Spencer.
Genius Foods holds a 53 per cent share of the gluten-free bread market in the UK and has grown the product category by 250 per cent since June 2009, according to figures from supermarket analysts at Kantar Worldpanel.
Roz Cuschieri, chief executive at Genius Foods, said: “Sir Bill brings a wealth of international experience to Genius and we are incredibly fortunate to have him on board as we strive to create a truly global food business.
“I should also like to thank Janice Gammell for her tireless efforts in developing this business since the very early days.”