Hmm.. The 'randomly' change places... Not really a big fan, but then again, it is such a staggering idea to get your head around it is hard to understand whether or not it will be ultimately useful.
I for one am against 'random' effects. Some numbers might help to clarify your idea... For example is it a 50% chance that a creature switches or a 25%? If I have 20 creatures in play, and my opponent only has 10, would I be right in saying that only 10 creatures are eligible to undergo the 'chance to swap'? If that is the case, how are the random 10 creatures chosen? Or will a random creature get swapped until 10 have changed places?
You see, it just throws up too many questions. I doubt it will enter testing. Nice idea though
.
As for High Gravity... Consider Scyther's input and Icybrakers questions. Gravity seems to cope best when dealing with 'large' or 'high defense' creatures. Example, Gravity Shield completely blocks anything with more than 5 hit points.
The way that your card reduces hit points works against this fundamental aspect of gravity. My suggestion would be to make it not a rare card, but a very much average card.
Then, make it work like so:
High Gravity
Cost:
Ability: Opposing creatures with hitpoints over 5 gain -1/-0.
Upgraded
Crushing Gravity
Cost:
Ability: Opposing creatures with hitpoints over 5 gain -2/-0.
Then again, you would still be a lot better off just using a Gravity Shield - which will completely block any damage coming from something over 5 hit points. My idea to make it a non-rare is that if you had 6 of these in play (upgraded), any creature your opponent has with 5 hitpoints or higher would get -12 attack.
My hoping is that -negative attacks are not affected by shields.
This way, you could build quite a synergy with Earth/Gravity (not sure more synergies between these two are advisable though, very powerful). Using Plate Armors to put your opponents creatures over 5hp, whacking out Crushing Gravities with Enchant Artifact, spamming a couple late game Stone Skins... You could potentially turn Earth/Gravity into a healing machine like Life/Light.
An alternative suggestion would be to make it a spell, that effects all enemy creatures in play the moment it is cast. This suggestion would work like so:
Gravity Flux
Cost:
Ability: Creatures over 5 hitpoints gain -2/-0. Creatures beneath 5 hitpoints gain -0/+2.
This would pretty much kill two birds with one stone, and if you were able to play 3 or 4 of these in a single turn against a full board - you could turn the game around quite significantly.
Anyways, don't take my input/criticism as a topic hi-jacking... I like the idea