Gravity modes control where particles in the simulation fall. These modes are not related to Newtonian Gravity - they let you change the default "fall down" behavior of particles.

There are three gravity modes:

On

This is the usual gravity mode - every particle falls down.

Here's a video displaying it.

Radial

This is an interesting gravity mode that makes everything fall towards the center. Particles might behave a bit weird and form other shapes instead of a sphere - just quirks of the particle engine.

Here's a video of the effect.

Off

There's no gravity in the save at all! Newtonian Gravity and air effects will still move particles around, though.