If you release the block from the spring's unstretched position, the change
in gravitational potential energy will equal the work done by the spring and,
as well, the change in the block's kinetic energy. The block will have some
velocity as it moves through the equilibium position.
To get the block to the equilibrium position you either let it oscillate many
times until kinetic energy is lost to heat in the flexing of the spring, or
you let it down very slowly with your hands. But in either case the original
gravitational potential energy is not simply the new elastic potential energy
in the spring.