Gravitational Field and Mass

As I know that the gravitational field on the surface of a neutron star has a relativistic mass density of 1.3*10⁵--1.9*10⁶kgm^-3 which is 100--1000 times the density of the water and is 10--100 times the largest mass density in the Earth, I am very astounded and wired. My first intuitional imagining is that this field could directly convert into mass. It is clear, the gravitational field exists in everywhere in our universe. No vacuum that the region of a space is “empty” does exist. A particle is clearly always being acted on by a gravitational field. The quantum vacuum fluctuation and vacuum polarization need be re-understood with the interaction between photon and gravitational field. Therefore, the gravitational field need be considered as one of the foundations of modern physics.