I've run a lot of RPGs, in a variety of settings. I don't think I've ever designed a setting's physics. I have sometimes considered a setting's sociology and/or theology and metaphysics, because to me these are far more central to RPGing than physics. But other times I have let those things emerge more-or-less organically via play; or have established them by reference to a known exemplar or paradigm (eg
Let's play Vikings or
Let's play Middle Earth or
Let's play Victorian-ear Cthulhu).
By gravity which of the following do you mean?
Things fall to earth - which has been common knowledge for all humans for as long as humans have existed; or,
All things fall to earth at a uniform rate of acceleration if air resistance is disregarded - which has not been common knowledge and as far as I know isn't obviously true in any FRPG I'm familiar with, given that none of them involve detailed calculations about falling things, ballistics etc; or,
The same physical factor that explains how and why things fall to earth also explains celestial motion - which is actively false in many FRPG settings.