An Interesting Aspect
I'm not sure if anyone has noticed this, but if you enter a negative value into a planet's gravity field, the planet will actually repel ships as opposed to attract them. Could be useful for planets you don't want players to get to if you put in a high enough value that makes it almost impossible to get near.
I discovered this by chance of experimenting with SFA. A few of the systems have 'gravity shear' listed as a navigational hazard. This struck me as odd since most ships in SFA are inertialess and therefore immune to gravity. I went to one such system to see how strong the fields were by launching the shuttle all Starfleet vessels are equipped with in SFA. Nothing happened (in hindsight, this also means ships susceptible to gravity in formation with a ship immune to it also become immune). When I had the shuttle hold position, it began to move—but away from the planet in the system I had selected. I checked the system in Mission Computer and found only one spöb in it. After checking the spöb, I noticed it has a gravity value of -1.
After a simple test, I confirmed negative values do cause spöbs to repel ships.
On a related note, gravity wells actually do get weaker as you move farther away from them, though I suspect that is widely known.