At what altitude above earth's surface would the gravitational acceleration be 9.0 m/s2

It would never be this value.

Acceleration due to gravity is a constant.

if F=ma

Then

a = F / m

which cancels down to give a fixed value of 9.8m/s^2

So regardless of mass, all free falling objects accelerate at the same rate.

(Look up on youtube Feather and cannon ball drop in a vacuum)