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.

Custom

You can adjust gravity up to 2.0 total in this gravity mode.