GSM doesnt mean unlocked. GSM is a network type. There is GSM and CDMA. Some carriers/service providers use GSM,like AT&T for example, and some use CDMA like say Sprint.
With Metro, well since they use other carrier's networks, it depends on what they are using in that area. In some they are CDMA and in some they are GSM. You would have to call Metro and ask them about using that device in your area.
However, before doing that, you would need to make sure the phone is unlocked. That part can only be done by the original owner and through the original carrier/service provider.