Why is this wrong, Spring Problem

A 2.00 kg block is attached to a spring of force constant 590 N/m as in Figure 7.10. The block is pulled 6.00 cm to the right of equilibrium and released from rest.

(a) Find the speed of the block as it passes through equilibrium if the horizontal surface is frictionless.

We can use potential energy here...MGH=1/2MV^2
2GH=V^2
2(9.8)(.06)=V^2
V=1.0844 m/s

Why is this off?

because it isn't gravitational potential energy... it's elastic potential energy.

The elastic potential energy is $$E=\frac{1}{2}k\Delta l^2$$