Gravity is generally considered OP. But I like how you structured it. Gots me a couple of questions though, before I go ahead and give this the go-ahead.
You speak of a "bubble" in which you have control, and the size of the bubble increases as your skill increases. You also have a "zone", which is the area affected. So can you only affect things within a certain distance (the bubble) and once you use the fruit, everything within the zone is affected? Can you specifically manipulate individual things like rocks or people, without affecting things within the zone? And lastly, can I see the bubble if I'm watching you use the fruit, like how the marines can see Law's Ope Ope no Mi bubble?
Personally I would treat it like Law's ability (if you know of it) where you have a bubble, and within the bubble you can do basically anything with the gravity dependent on your control. But, if you did that, since your potential control is increased, I would insist the bubble is visible.
yes the bubble would be visible like law's, sorry thought i put that in there and it is possible to affect only certain things but i would make that probably say he couldnt until rank 3 or 4 to make it fair and it also would have to take more concentration than to just affect everything. Also here is a little reasoning behind having the "zone" AND the "bubble", i want to be able to affect myself with certain gravities but not all of them so if i make it like i did the gravity does not have to be centered around me therefore i could affect someone else with a downward gravity without getting myself caught in it or i could center it around me and make it affect myself also.
also i was hoping to add in that after rank 3 he can center the "zone" around something so that as long as its withing his bubble it will move with the object, but the object can only be inanimate or himself
I still don't see the reason for the zone. It's not clear how it could be used properly.
Just make an bubble that you have control within. The biggest thing for me is the zone is consistently the same size of the bubble, ie. 10 feet by 10 feet by 10 feet when your bubble is only a 10 foot radius. So the way you've explained it means that when you activate the zone, it will affect everything in your bubble anyways.
Just make it a bubble that you have gravity control in. That way you can manipulate individual things without affecting yourself. The zone just doesn't make sense and is too complicated, which always slows RP. I'll approve that, but probably not much else. Unless you somehow explain it really well.