G is the gravitational constant, the m’s are the masses in question, and F is the force generated. The r is radius from the center of one body to the other; that is, height. If it didn’t decrease, orbits wouldn’t exist the same way and astronomers would have laughed Newton out of the room.
I could give you a link if you really want, but it’s the Newtonian gravity equation, so it’s probably just going to be “Gravity” on Wikipedia.
G is also fixed in GR, although it’s not guaranteed to manifest in a neat relation like that in every situation because spacetime curvature has a lot of components at every point, and they interact super nonlinearly.