I don't see how gravity or aether fit with the idea. It certainly isn't as...beefy as a normal gravity creature, either, and I dislike the idea that a card should be designed around the needs of the game, rather than for the theme of the card and the elements.
Entropy does fit to some extent. It fits with the idea that as energy is used, it spreads out, becomes less concentrated. This absorbs the energy that creatures put into using their skills to get stronger, so I can see that fitting well with the card idea.
However, the result of all of this, for the universe, is heat death, which is quite a fire type term.
I still like it most as fire. Weak creatures that can get strong, and strong/small creatures fit well with fire, and this is both, but I can imagine how entropy would work.