What does it mean that gravitation separated from the four forces at $10^{-36}$ seconds? Physicists have used math to calculate that gravitation separated from the 4 forces at 10^-36 seconds after the big bang, which is even before the inflationary period.  The universe was still smaller than a pin head at this point, so what does it mean?  Could it be better described that spacetime as we know it took shape? (regardless of if you are proponent of string theory or loop quantum gravity).
I have read this question, but I am really only concerned with gravitation and the formation of spacetime, so I am not looking at the other forces that separated later.
 A: This is really a long comment.
Lets see the concept of force:

One of the foundation concepts of physics, a force may be thought of as any influence which tends to change the motion of an object. Our present understanding is that there are four fundamental forces in the universe, the gravity force, the nuclear weak force, the electromagnetic force, and the nuclear strong force in ascending order of strength

So you are talking of fundamental forces .
Gravitation and electromagnetism were considered forces within the Newtonian classical mechanics formulation. The weak and strong belong to the quantum mechanical realm where, in the quantum field theoretical calculations for particles, force is the  dp/dt the change in momentum in interactions of particles
When accepting there are four fundamental forces, including within this frame gravity one makes the assumption that quantization of gravity is a definite fact. This is not true, there are only effective theories for the quantization of gravity, used in cosmological models ad hoc to give the statement you make

Physicists have used math to calculate that gravitation separated from the 4 forces at $10^{-36}$ seconds after the big bang,

The three forces, weak, electromagnetism, strong  can mathematically be extrapolated to the beginning of the big bang with Grand Unified Theories with mathematical rigor, they embed the standard model of particle physics.
Trying to include gravity for those high energies, as you also state, is a matter of models that are not yet mathematically rigorous in modeling gravity and embedding the standard model of particle physics., (as string theory and loop quantum gravity).
In my opinion, the grand unification epoch is still a hypothesis used to model the beginning times of the universe in a hand waving way, until and if a theory of everything is developed. The numbers are based on fitting cosmological observations with simple reasonable assumptions .
This essay might interest you.
