Inertia has nothing nothing to do with acceleration or gravity, strictly. Inertia is just velocity*mass, relative to an arbitrary rest frame.
I really must dispute that position Fort.
Inertia is (still) defined as a resistance to acceleration, that's a basic textbook definition so that's hardly "nothing to do with" acceleration.
The product of velocity and mass is known as
momentum, not inertia. Prior to Einstein inertia was regarded as absolute, not relative because the inertia experienced is the same no matter which reference frame we measure the acceleration relative to.
Einstein postulated that it is however relative to the universe itself and this is more or less Mach's principle (he was an admirer of Ernst Mach).
Finally Einstein assumed (it is axiomatic) that inertial mass and gravitational mass are each described by an identical law, that law is expressed (using a very compact notation) in the Einstein
field equations. These define a tensor field (as opposed to a vector field).
The
field is the same form whether we are considering a body at rest under gravity or a body accelerated without any gravity.
I could talk more about this, but wanted to emphasize this point.
