Gravity is very important for having the highest battle power possible. To use gravity you need a gravity machine which is made thru science. When you turn the machine on a gravity field will be emitted within a nearby area and anyone within it will be under the pressure of it.
If the gravity field has higher gravity than what your character has mastered then it will cause damage. This is good, because if it isnt causing damage then you arent mastering it.
The more gravity you have mastered the faster your battle power gains will be. The more damage the gravity does to you the faster you will master it. Do not get knocked out in gravity because you will cease to master it while knocked out.
Using the train command in the gravity causes you to master it 1.5x faster.
Sparring in the gravity causes you to master it 5x faster.
Gravity is EXTREMELY expensive and rare. You can leech a person's gravity mastery by sparring them or thru zenkai. See the "power adaptation" section for info.