Quote from the game:
"Aether is the domain of the immobile things; it is where the stars reside and the media where all the energy moves." "Gravity elementals deal with mass and size..."
So either fits. I believe its commonly agreed that aether is equivalent to space based on that definition and the fact that it is the opposite of time.
As a gravity card, gravity activation, to make a light and fire producing permanent we could have Trio decks using Armagio, Graviton fireeaters, rage pots and archangels, logically targeted for destruction with little gain (recommend changing nova/supernova to blackhole possibly to make the decision to destroy it more strategic)
As an aether card, gravity activation, to make a light and fire producing permanent it would most likely have to go in a rainbow hoping the AI would be dumb and destroy it for the nova (weak card in pvp, average over all, maybe a rainbow heavy in fire and light)
Based on that, I vote gravity.