I like the card, but I don't like how the card isn't compatible with Gravity creatures. Surely there's a way you could make it so a Gravity card is usable in a mono-Gravity deck? Capping the attack post-switch to 20 or something?
There's a few reasons gravity creatures aren't affected.
1. The highest HP creatures in the game are all gravity. 70|7 flying titan = OP. Even 30|8 = OP.
2. In my mind, these creatures "wield" gravity, so they shouldn't be affected by changes in it. I'm just weird though XD
3. People clearly like the card as it is, because it got to level 2, and doing good in the voting still.
I am still toying with the idea of making it so it can target any creature with less than 20 HP. But I'm still not sure.