First off, have you bled the coolant to make sure you don't have air trapped in the coolant passages?

Before assuming the gasket has failed, I would confirm that:
a) the radiator cap is maintaining pressure
b) the thermostat is opening fully and
c) that the thermostatic fan switch is functioning. The radiator fan should be coming on as coolant temp hits 202° to 205°F.

Also, when you pin the throttle, is there any condensing steam coming out of the exhaust?

It is possible that the new head gasket was improperly installed or that the head bolts weren't properly torqued. Both would lead to coolant seeping into the combustion chamber, causing the symptoms you describe.