There's a little game, forget its name now, that comes with (or, of course is package-manager-equiv addable to) various Linux distros, with a number of 'planets' and two firing platforms (1P+CPU or 2P) that angle and speed their shots to slingshot... (it's not called Slingshot is it?)... their bullets to hit the other firing platform through the various gravity wells in-between. Might be worth trying to track down what it's called. It'd be free/GPL/whatever.
And xpilot, of times past, could have gravity wells (and anti-gravity ones) dotted around the 2D environment, so you could make a level with a whole bunch of gravity wells in clumps surrounded by killing-tiles (or at least overwhelmingly damaging ones) which made for an interesting navigational exercise by the Asteroids-style space-ships (and their bullets/missiles/etc). Haven't played that for... 18-20 years, though. No idea if it's still around, or changed beyond all recognition. Also probably GNU-ish. If not then, then certainly by now if it is community maintained code.
(BTW, I'm sure more people misconceive it as something to do with an atmosphere ((as in, suck the air out of a chamber and the suited astronaut inside will no longer be subject to Earth's gravity... or so some fiction would have you believe[1])) than associate it with a flat plane. But I know what you mean...)
[1] Closely related, I suppose, to the whole fictional representation of being on a ship (or even moon-base) with artificial gravity (usually exotic and non-centripetal, but sometimes even then!) but starting to float (or experience 1/6th gravity) when your airlock is evacuated... That's probably something to do with the "gravity plating" under the airlock, or whatever, being phased off at the same time as the air is thinned, though. Logical enough to do, if you had the facility to do that.