The Universal Gravitational Constant

The universal gravitational constant is called G because it's supposed to be the same all over the universe. If we lived in a universe where G was twice as big or if the cartoonist at the control panel fell asleep on the knob and doubled big G, then all the forces of gravity would be twice as big. And one of Newton's great achievements was using this to describe gravity here on earth between fairly small masses and small distances. It also makes sense that it's one over the distance squared because as things get further and further apart, the force gets diluted over a larger area.