The Gravity space warp simulates the effect of natural gravity on particles generated by a particle system. Gravity is directional. Particles moving in the direction of the gravity arrow accelerate. Particles moving against the arrow decelerate.

Particles falling because of gravity
In the case of spherical gravity, motion is toward the icon.

Gravity effect on snow
Create panel, click
(Space Warps). Choose Forces from the list, then on the Object Type rollout, click Gravity.
The Gravity icon appears. For planar gravity (the default), the icon is a wireframe square with a direction arrow on one side. For spherical gravity, the icon is a wireframe sphere.
The initial direction of planar gravity is along the negative Z axis of the construction grid that is active in the viewport where you drag. You can rotate the gravity object to change the direction.

Increasing Strength increases the effect of gravity; that is, how objects move in relation to the Gravity icon's direction arrow. Strength less than 0.0 creates negative gravity, which repels particles moving in the same direction and attracts particles moving in the opposite direction. When Strength is set to 0.0, the Gravity space warp has no effect.