Let's talk about the gravitational constant, /sci/.

According to Einstein the following hold:
1. Gravity is not a force but merely the curvature of spacetime
2. Mass is equivalent to energy, the binding energy of for example gluons and quarks amounts for much more of the proton or neutron mass than the masses of the particles themselves.
3. (relativistic) mass depends on the velocity of an object

Then how can Newtonian gravitational force be formulated simply by multiplying both rest masses by a constant? Shouldn't G itself depend on
1. the geometry of spacetime
2. the type of mass (particle mass vs energetic mass)
3. the velocity of the particles
?

Also why can't G couple with photons even though they are affected by gravity?