Overheating

I have EX model 2.3L. Now here is my problem: Not around town or at idle but on the highway it overheats. I only let the temperature gauge it get to 3/4 of the way hot before I turn the heater full blast and that cools it right down to under the half way point on the gauge. I turn the heater off and it goes right back up. This is what I have done to try to fix it. I installed a new thermostat. Did not fix it. Flushed the whole system Did not fix it. Installed a new fan sensor Did not fix it. Hard wired both fans to run all the time Did not fix it. Installed new timing belt and water pump Did not fix it. Installed new radiator Did not fix. Now they are telling me it must be a bad head gasket? Do you think that is my problem? If not what else could it be? If it is can I do it? I have done all the rest so far on the car. Can any one help me with instructions on how to do this or send my a link to install the head gasket?

Thanks for taking the time to try to help.
Brian