Yeah, you'll need a lot of variable work and battle commands in every monster group in your game, but it's possible.
At the beginning of your project, set variables equal to each of your characters' max HP (Under the "Hero" Operand), then in every M. Group, set a trigger that when your gravity spell is used, it reduces everyone in the party's HP by that variable.
That's a really vague outline & it requires a lot of work, but you can get it functioning if you spend some time on it.