How does one characterize the strength of a force? Force has to act on something. This thing is called a "charge".
I don't think you should take this question too seriously.
Gravity acts on mass. The nuclear force binds nucleons together. To get some sort of comparison you need a typical particle mass and the color charge between quarks.
From what I've been able to decipher, it's an artificial comparison where colour charge of a particle is pretended to be equivalent to its gravitation charge.