Yeah kirk and ACES said it all lol.
That Gravity Generator options in Brex's menu are to control your grav generators, if your ship has one. To add one to a ship you gotta add it in the hardpoint, just like they told ya. And also like they said, GravFX's documentation has all the info you'll need about what to do to correctly add a grav or anti-grav generator in the HP. It just doesn't tell how to use MPE, but just search the forums around here and you'll find that info.
As an example: if you went to the Psi System, there's a blackhole there. That system is part of GravFX, to show how a working blackhole could be made in BC. The blackhole itself is a ship, and as such has a hardpoint, with a very powerfull grav generator, so check it if you want to see a HP with a grav generator. However in the game's /hardpoints folder all HPs from KM will probably be in the un-readable .PYC format, but KM should have the readable .PY version of all HPs in its /src folder if i'm not mistaken, otherwise ust ask for it around here if you want to check the blackhole's hardpoint.
GravFX's readme states this but i'll say it again just so that it is clear: the gravity force applied by GravFX affects only ships following real physics formulas and all. Anti-gravity force works the same way, however it's vector is inverted (to be anti-grav), and it'll also affect projectiles (most likely torpedoes since they are slower, thus possibly acting like a anti-torpedo shield, which by the way is really nice lol).
It will not prevent ships from warping out.
(However since i'm in a nice mood I might add something like that in Galaxy Charts v2
)
A ship's grav well will not counter a planet's well thus making the ship "unaffected". The ship will still be pulled by the planet. The planet won't be pulled by the ship because for technical reasons GravityFX won't affect planets. However a second ship in the area will be pulled by both the planet and the first ship.
Just remember your physics (or math) lessons: addition of vectors. The gravity force is nothing but a vector, so that second ship will be pulled in the direction of the sum of both grav force vectors acting on her.