The acceleration due to gravity on the surface of the earth is given by, g=R2GM.
The radius shrunk by 2. Therefore, RΔR×100=−2.
As the mass is constant and the change in the radius of the earth is relatively small, we can write
gΔg=−2RΔR⇒gΔg×100=4
Hence, the acceleration due to gravity increases by 4.