So I have really been pondering this question for the past few weeks to find the relationship between this constant 'G' and its relationship in the equation for the force of gravity between two massive bodies. The equation for a proportionality relationship is y = kx. This formula looks familiar because Hooke's law is nearly identical to this except 'k' is made negative (not sure why but perhaps because spring rest is 0 in cartesian plane and spring force direction opposite of x?). Anyway, 'k' represents a constant and a constant is a variable used to show a relationship between two or more coordinates. In our case with gravity, we want to see how the force of gravity increases or decreases between any two massive bodies. The two factors that have significance in changing our gravitational force are distance and mass. *The strength of the force of gravity changes at a ratio of 6.673 * 10^-11 for every kilogram of mass with respect to the distance in meters (squared). Basically, this gravitational constant tells us by exactly what interval the force of gravity between these two massive bodies changes. F = mg is just like the equation y = kx. Its the equation for proportionality between two coordinates. this formula is simplified because g = GM/r^2. I'm still not very sure how Newton could have derived the formula F = (M1 * m2) / r^2 without the gravitational constant, because to me it still seems you would need a constant of proportionality in order to calculate exactly by what factor the mass and distance change the force of gravity between the two bodies. It seems that without that number you can understand the relationship among mass and distance and the force of gravity but no actual calculations could be obtained without a constant. Perhaps this could be due to my currently low understanding of different units systems. Some final thoughts I had on the force of gravity: I started wondering why we don't use an intrinsic gravity number in which we can say this planet has a mass of 'x' kg therefore it has an intrinsic gravitational pull of 'x' newtons, similarly to voltage in a battery. But then I remembered that a potential difference, which is the potential energy between two points, constitutes the number representing voltage for a battery and the current is more important than the voltage anyway. But I came to the conclusion that maybe we don't have this intrinsic number because mass in itself is already somewhat of an "intrinsic "gravitational number." Mass tells you that if the number is really high there will be lots of gravity. So gravity is a number that will tell you the influence it has on a specific object which is incredibly useful. I hope what I have written was clear enough to understand my interpretations on the variables associated with the formula for gravitational force.
* I put an asterix by this sentence because this phrase here brought up a inquiry for me. How can we set this constant to m^3/(s^2kg) just to get the units to cancel? Why should the units need extra help to get cancelling from what should be a unitless ratio?