How about something like this:
During gravel/bedrock's turn (when it "attacks," which it doesn't -- 0 attack), if there are any creatures with gravity pull, gravel/bedrock is destroyed and the creature with gravity pull is buried for 5 turns (attack is halved, health is thirded, rounded down, can't attack or use abilities, can't be targeted). All creatures on the same level (horizontal line) lose half (rounded up) health and become buried (attack is halved, can't be targeted, can't attack or use abilities, gain ability "unburrow" for 0, which subsequently turns into whichever ability they had before, and doubles their attack.