I can't think of ANYTHING good to be achieved by blocking off part of the cooling during a head gasket replacement. I really hope he didn't block cooling passages with the gasket.
I'm can't see how blocking cooling anywhere would make your gauge react slower. A thermostat blocked open could let the coolant take longer to reach operating temperature, if at all. That's not a good idea either!
It took a long time for our diesel to reach 85 this morning with a proper thermostat, but it was 0F outside! With an open thermostat we would still be waiting for heat.
Personally, I'm stumped! Perhaps he didn't do a good job of explaining what he did.