When two bodies have masses equal to 1 Kg and the distance between them is 1 m, then the value of gravitational force is equal to the gravitational constant. i.e. 

$$F = \frac {G m_1 m_2}{r^2}$$

$F$ is the gravitational force, $G$ is the gravitational constant, $m_1$ & $m_2$ are the masses of two heavenly bodies and $r$ is the distance between the two heavenly bodies.

When $m_1 = m_2 = 1$Kg  and $r = 1 $m,

$$F = \frac {G. 1} {1} $$

$$\therefore F = G$$