The Gravity is a FumeFX helper object, but it functions differently than previously mentioned Sources. You can attach one of these helpers to a FumeFX grid in order to direct gravitational (and buoyant) forces in a direction other than global Z-axis or to use it as point gravity.
Type - determines the shape of the gravity field. Vector type allows you to alter the direction of the gravitational pull. With the Point gravity option, the Gravity helper object will attract smoke and fuel towards its center. Keep in mind that with the Point gravity, falloff is proportional to the squared distance like in the real world. In large scenes you might need to increase the strength significantly to see any influence.