What I was trying to say is this: we understand electromagnetism very well, to the point that we can generate the electromagnetic constant (the permittivity of a vacuum, most of the time) by performing arithmetical operations on known properties and quantities. We understand how these things interact.
That isn't the case with G: we know how big it is, but that's all. We know the magnitude of the number, and we know the units we measure it in, but we don't know why.
We can't point at physical properties and say, "This multiplied by that, and divided by these two things, makes G what it is." We can do that with the electromagnetic constant; we can't with gravity.
I'm convinced that understanding gravity to that extent would enable us to unlock the secret of gravity control. Unfortunately (as I said in the linked post) gravity is such a weak force it's hard to experiment with. In fact, EM is the only one of the four fundamental forces which is approximately man-sized, operating on a scale humans can easily comprehend.
The weak force is much more powerful than EM, but it operates within an atom's nucleus, and we didn't learn much about it until just seventy years ago--and while we understand it well enough to be able to control the rate at which fissionable materials fission, we can't make it sit up and beg the way we can with EM. Yet we've got a pretty good idea of how EM and the weak force can be unified, to the point that some physicists now talk about the "electroweak force".
Then there's the strong force. It operates on a scale commensurate with the diameter of a proton, and it's the strongest force there is. It's what binds quarks together to make particles. It requires so much energy to separate quarks that when you finally separate them farther than the strong force can handle, new quarks are created and you suddenly have several particles where you once had only one or two. (Proving Einstein's most famous equation, the mass-energy equivalence, in the bargain.) The strong force requires such titanic energies to explore that we have to build gigantic machines to work with it. Example: the Large Hadron Collider built by CERN. (The so-called "Black Hole Machine".)
It's possible that there might be another force, even stronger and working on an even smaller scale. There's a theory that inside quarks are "preons"; and if there are, there'd have to be some force holding them together. I don't know if there's enough energy in the universe to force them apart, though.
The important thing is that these forces are mediated with particles. For EM, the mediating particle is the photon. For the strong force, it's the gluon. (Not sure what the weak force uses, unless it also uses photons.) But we haven't found such a particle for gravity.
Quantum gravity is the holy grail of the standard model, but no one can make it work. The work being done at LHC to find the Higgs boson might reveal how mass becomes "massy", but Hawking's bet a Franklin that they won't find any evidence of the Higgs boson...and Hawking is not exactly a dim bulb.
If gravity can't be quantized, what then? Does it mean gravity control is impossible? Damned if I know. But it's fun to think about.