How general relativity gets to an inverse-square law I understand that a general interpretation of the $1/r^2$ interactions is that virtual particles are exchanged, and to conserve their flux through spheres of different radii, one must assume the inverse-square law. This fundamentally relies on the 3D nature of space.
General relativity does not suppose that zero-mass particles exchanged. What is the interpretation, in GR, of the $1/r^2$ law for gravity? Is it come sort of flux that is conserved as well? Is it a postulate?
Note that I am not really interested in a complete derivation (I don't know GR enough). A physical interpretation would be better.
Related question: Is Newton's Law of Gravity consistent with General Relativity?
 A: 
I understand that a general interpretation of the $1/r^2$ interactions is that virtual particles are exchanged [...] General relativity does not suppose that zero-mass particles exchanged.

You don't need quantum field theory for this. In a purely classical field theory, we have field lines, and the field lines from a spherically symmetric source should radiate outward along straight lines. In a frame where the source is at rest, we expect by symmetry that the field lines are uniformly distributed in all directions. The strength of the field is proportional to the density of the lines, which falls off like $1/r^2$ in a three-dimensional space.
This whole description is complicated by the polarization of the field. Gravitational fields have complicated polarization modes. Nevertheless, the $1/r^2$ result is unaffected.
Finally, we have an issue unique to GR, which is that the field is the metric, and this means that the field itself affects the measuring tools that we use to measure things like $r$, the field, and the area of a surface through which we're counting the number of field lines that penetrate. These are all strong-field issues, so for large $r$, they don't affect the $1/r^2$ argument.

Is it a postulate?

No. In the standard formulation of GR, the main postulate is the Einstein field equations. From it, we can prove Birkhoff's theorem, which says that the Schwarzschild metric is the external field of a static, spherically symmetric source. The weak-field limit of the Schwarzschild metric corresponds to a $1/r^2$ field.
A: I found many explanations for this type of questions
http://settheory.net/cosmology
http://settheory.net/general-relativity
It's better than "The Meaning of Einstein's Equation" (John Baez).
In particular
 - It is directly applied to an important example (universal expansion)
 - The expression is simpler (relating 1 component of the energy tensor
 to 3 components of the Riemann tensor)
 - The relation between energy and curvature is not only expressed but
 also justified
 - Both (diagonal) space and time components of the relation are
 expressed and justified, resulting in showing their similarity "like a
 coincidence".
