Yoshi P made it very clear at the london fan festival that the next class would be using a gun. SE has also said that the next major expansion would feature both a tank and a healer, leading many to believe that the gunner class will be a healer. As odd as this sounds, there are several classes with healing capabilities that use guns in the final fantasy series. Chemist from FFTactics, and Agent from FFTactics A2 both had some of these capabilities. Gun mage and Alchemist from FFX-2 also had strong healing components.

Right now the Agent class is heavily favored because Yoshi P hinted at the class by wearing a james bond 007 shirt, and he is a secret "agent"

Time will tell it seems, but expect it to be live in 3.0 Heavensward. They will probably reveal the class in the next FF scheduled for later this year. At the American FF (a week ago), Yoshi P wore a batman (dark knight) shirt to hint at the class, before officially revealing it at the london FF (2 days ago)