It sounds like the previous owner let the engine overheat (reason possibly for the bad head gasket) and now you have inherited his problem. Whoever did the head gasket job may have missed something, like a cracked or warped head. It depends on the severity of the overheating, and since you don't know, proceed carefully. There are several tests you could have done: check for exhaust gases in the coolant-a simple test using a colored fluid that turns yellow if exhaust gas (hydrocarbons) is in the coolant, you could have the coolant system pressure tested to check for leaks and to make sure that system holds pressure, and you could have an engine compression test done to check for an internal mechanical problem-low compression in two adjacent cylinders is a head gasket leaking, possibly a warped head.
I would do the compression test. Knowing the engine is good and has good compression (translates into good power), means you could go ahead and put money into repairs.
Did you change the water pump when you changed the timing belt? That would have been a good idea. You might want to have the coolant flow rate checked, see if the water pump is pushing coolant good.

I don't see any mention of having the cooling system flushed or the water pump being serviced. Both could be creating your problem. Also are you sure the new head gasket was installed properly, the right one and not upside down.