Why is G negative in Newton's formula of universal gravitation?

In the formula F = -G*m1m2/d^2, what is the significance of -G? I know sometimes the G is positive, so what is it denoting when it is negative? Obviously it's making the value of m1m2/d^2 decrease rather than increase when distance decrease.... I guess what I really want to know is what does m1m2/d^2 represent? Is it gravitational potential energy?