Gravity is technically an element mechanics-wise- it's just that it has way less sources. I guess it's tougher to work a fundamental law of nonmagical physics into spells and potions.
Percentage-based damage has always been one of those effects in, say, Final Fantasy that's sometimes really overpowered in a few specific scenarios, and then never works on anything you'd really want to use it on. I wanted to do something different with Gravity, which is why I made the Grav Formulas work the way they do- a nice help against big blobs of HP, but doesn't (usually) do enough to one-hit KO or completely trivialize everything, and falls off against foes with self-healing.
Against someone without the skillset to counter him properly, Yg'Xel would be terrifying. On the other hand, a Challenger Bone Mage could probably ruin his day with a proper strategy, stat disparity or not! If he wants to be taken seriously, he'll have to learn from the smackdown Brill just gave him.
Good night!