Is Hubbles law due to Gravity? Hubble's law states that Distance is  proportional to Velocity. A ScienceDirect article states that Classical Hubble expansion is characterized by a proportional increase in the rate of expansion groups based on the distance from the main center of gravity
So  is it due to gravity?
 A: No, Hubble's law is due to the expansion of the universe.
I can't say what the ScienceDirect article meant because you didn't link it, but the description of what it wrote does not sound sensible either.
Edit: from the link, the relevant quote is:

A special place in astrophysics is the effect of Hubble—expanding groups of galaxies, accompanied by a proportional increase in the rate of expansion groups based on the distance from the main center of gravity.

What this means is that if you have a group of galaxies,* then when space expands, the galaxies appear to recede from one another at a speed given by the Hubble Law. The Hubble Law itself is still caused by expansion of space.
*Note "groups of galaxies" (i.e. galaxy clusters) are gravitationally bound - that is, they do not expand because their gravitational attraction is strong enough to overrule cosmic expansion.
A: Hubble's law describes the expansion of the universe. Is it due to gravity? Well gravity is the interaction by which space time responds to energy soruces (and therefore does anything at all), so yes. But you can't just say that any hypothetical universe containing gravity will therefore automatically obey Hubble's law.
This depends on the constituents of the universe and also which "part of spacetime" you're interested in. There are some details about the present epoch (such as matter dominating over radiation) which have been shown to inevitably follow from more basic properties of the early universe. But there are also plenty where we are far from having a theoretical explanation. One of these is dark energy which has a significant effect on what Hubble's law will look like in the future.
