Gravitational Field Strength on the earth’s surface is defined as the force per unit mass experienced by a small test mass placed on the earth’s surface (in the gravitational field of the earth).
So the Gravitational Field Strength at this point is expressed by the formula, g = G M/r^2
where M = mass of the earth =5.97 x 1024 kg,
r = radius of the Earth = 6367 km,
G = Universal Gravitational constant=6.67 x 10-11 N m2 kg-2
[ visit this link to see the derivation of the Gravitational Field Strength formula]
Now putting all the numerical values in the equation we get the Gravitational Field Strength on the earth’s surface = g = G M/r^2 = 9.8 N/Kg
Observation: This is the same as the acceleration due to gravity, which is what is expected since Newton’s 2nd law says a = F/m = force per unit mass