We know that,
Force = Mass × Acceleration
F=m×a
a=Fm ...........1
Where F is the force on the object of mass m dropped from a distance r from the centre of earth of mass M.
So, force exerted by the earth on the object is
F=GM×mr2 ..........2
M = Mass of Earth
m = mass of object
r = distance of object from centre of earth
Now, from equation 1 and 2,
a=GM×mr2×m
a=GMr2
Now, from above,
a=g = Acceleration due to gravity
We also see that, although force is depending on the mass of the object,
F=GM×mr2
But acceleration due to gravity is independent of the mass.