pmb

There are a lot of assertions in your post. I'll approach this by addressing them one by one.

Lately i have seen people theorize that gravity is not a force and it is a natural motion because gravity pulls something instantly.

Who are these "people" you are refering to? What do they mean by "gravity is not a force"? What do you mean when you use the phrase "natural motion"?

Like if you were to drop something it does not have any delay before it starts to fall.

I don't see a problem with that? This is where the idea of a field came into use in physics. Regarding your example, a similar thing happens when dropping a charged body in an electromagnetid field. The way these phenomena are described in modern physics is that as follows:

Source generates field at R = R(x,y,z) -> field acts on body which is located at R = R(x,y,z). Body at R = R(x,y,z) accelerates according to field strength at R = R(x,y,z).

In the case of gravity the source is anything that has matter, i.e. something with non-zero active gravitational mass.

In the case of electrdynamics the source of the electromagnetic field is anything which has charge and/or current.

Sorry but I have to cut this off here. I keep falling asleep at my desk.

Pete

It depends: in Newtonian gravity it is a force as the electromagnetic force. In general relativity it cannot be a force, because this is a geometric theory.

 

What we know is that the fundamental nature of gravity has to be quantum and in a quantum formulation the concept of interaction is not reduced to the classical concept of force.

It cannot be correctly argued that "gravity is not a force" just because it is a geometric theory.

From Albert Einstein, in a letter to Lincoln Barnett (1948), wrote

This is not internal energy, but rest (mechanical) energy. Internal energy U is a different kind of energy.

Wow! It's as though this this thread was put together by a chain saw!!

First off the E in the expression E = mc^2 applies to particles and has the value E = Rest Energy + Kinetic Energy. The expression E = mc^2 appeared in the physics literature before Einstein put it there. But this is not a thread about history so I won't get into it but point you to the physics literature on this point. See the American Journal of Physics

Did Einstein really discover "E = mc²"?, W.L. Fadner, Am. J. Phys. 56(2), February 1988

In 1952, Herbert Ives claimed that Einstein's first development of E = mc² was circular, and that he had not been the first to develop that equation. That allegation has been repeated in several more recent works. Earlier, Planck asserted that one of the postulates that Einstein had used in that development was not exact. Those claims and subsequent papers concerning them are examined herein. The surprisingly long history history of the mass-energy relation is summarized. In the context of this topic, it is argued that circularity is seldom a legitimate critique of scientific proposals. A simple refutation of Planck's claim is also included.

JP wrote

That's crap put out by crackpots who for one reason or another want to knock Einstein down a notch or ten.

That's not true. This debate appears in the peer reviewed physics literature, for exaample see the above journal, i.e. the American Journal of Physics.

Regarding what E represents depends on whether the body is moving or not.