I had a slow leak of coolant and had the head gasket, water pump replaced. Today, when driving it, the coolant was overflowing thru the reservoir hose and it sounded like the coolant was getting stuck at the thermostat...

The landy has been running fine for two weeks since leaving the mechanic...

is this problem a bad thermostat that is not opening up or could it be the water pump that is not moving the coolant around?

 

It could be either, take it back, dont wait, take it back tommorrow as youi by law have a 30 day warrenty on all work that was done.
You also have whatever warrenty the parts came with, usually 90 days, sometimes more.

 

Could just be an air pocket heating up and pushing it out, Take it back ASAP before you overheat it and kill the engine...