Mass is not equivalent for gravity and inertia. Mass causes inertia. Matter's volume causes gravity. Mass is the area of mass, so it cannot cause gravity.
Matter shrinks space and grows time. That causes a gradient in continuum. Mass is affected by the gradient in continuum by accelerating. The area of mass is accelerated by the gradient in 4D continuum.
Here area the new Laws of Continuum:
http://fcgravity.blogspot.com/p/first-law-of-difffusion-of-herenowium.html#essay5