The agency will now work with the British Heart Foundation, Cancer Research UK and Diabetes UK.
It picked up the business following a pitch against the charities' incumbent agencies - Grey, which works with the BHF; Ogilvy, which looks after Cancer Research; and M&C Saatchi, which handles the Change4Life business.
The Gate will be briefed to create a campaign highlighting the link between lifestyle and diet with heart disease, cancer and type 2 diabetes.
The jointly branded campaign will also target parents with children under 11 to raise awareness that a healthy diet and active lifestyle can reduce the risk of developing these diseases later in life.
A spokesman for the partnership said: "We believe that by working together, we can help parents bring about the healthy lifestyle changes necessary to improve the long-term health of their children."