I remember I was testing out a planet mod I was making for a DM, and I had used this method and I didn't have any of the problems you stated, for example, I went into a zone with regular gravity, hoping that it would be applied, but I still was in zero gravity from the zone I gave the player.
Also I am certain the zone was smaller then the player object.
That's because 1 times 0 happens to be 0. When in two physical zones, your gravity is a product of the two. I was referring to low gravity, not zero.That's not an issue in and of itself. The problem occurred when you place the zone within the player, not when you walked through a small zone.
NoEdit: I was mistaken, it was a 4x cube.Perhaps just search if a player object is near, then share a cube between the two instead of overlapping? (Assuming they have the same gravity, which they should)