Is it much like the nature of light? I mean we can't completely define gravity but we do have some theories which cover more or less about the phenomenon caused by gravity. Similar to light, we accept that it has dual nature because there is no other theory which best explains all the properties of light.Gravity is an abstract concept. There is not "2 gravity" or "3 gravity". You cannot count a concept.If you would move half of Earth to somewhere else, the total mass of Earth would be the same, but you would feel a different gravitational force.For a point-like object, it works, for an extended object, it does not work.
There is no "truth" in physics, only good descriptions. Einstein's general relativity is the best model we have. Newton's laws are a good approximation to general relativity for all everyday situations.
Also there are no proper units to explain the gravity experienced by an object in a universal scale... correct?
Then the law, g=Gm1m2/r^2 wont be applicable in all situations, right.
Basically we don't really know gravity but we use that equation just because it can help us to a certain degree?