G and g are both related to gravitation.
G is the universal gravitational constant having the value 6.67×10⁻¹¹ Nm²/kg². As the name suggests, it is a constant and its value always remains the same.
g is acceleration due to gravity. Every massive celestial bodies have their own acceleration due to gravity and it varies with altitude from the centre of the body. For earth, the approximate value of g is 9.8 m/s² near equator and 10 m/s² near pole.