The appointment is part of Diabetes UK's ambitious plans to double its revenue and increase its support base over the next three years.
It followed the arrival of Mike Hales as director of individual relationships and marketing in May.
Hales said that the charity needed to develop greater awareness of what it does for diabetics and what it has achieved in research.
Jeremy Prescot, client services director at The Gate, said: "After successful campaigns in 2006 and 2007, we are very knowledgeable about how to reach people with diabetes and look forward to making the 2008 advertising campaign the biggest and most successful campaign."
Mike Hales, director of individual relationships at Diabetes UK, said: "It was an easy decision in the end. The Gate are streets ahead in terms of understanding what we stand for as a charity and will help us deliver on our ambitious marketing targets for the next three years."
Diabetes UK estimates that up to 750,000 people in the UK are unaware that they have diabetes, and risk blindness, heart disease, kidney failure and amputation.
The Gate is part of Gate Worldwide, an international agency network with offices in London, Edinburgh, New York, Hong Kong, South Africa, Shanghai and Singapore.