If I were to do a basic cylinder compression test (not leak down) would that help me determine if I have a bad head gasket?

Right now my blazer has a p0300 and no other codes and also starts to over heat after 30 minutes or so on the road. I haven't been able to check if there is any bubbles in the radiator. Well I take that back, there are bubbles but I chalked that up to being air in the system from starting it cold.

 

A compression test by itself will not confirm a bad head gasket. A compression test will determine if the cylinders can build pressure, and how much. Compression & cylinder leakage tests mainly target pistons, piston rings, and valves. If you're suspecting a leak in a head gasket between the combustion chamber and a coolant passage, the way to test for that is to install a cooling system pressure tester on the radiator, and pressurize each cylinder with compressed air. If there is an increase in pressure on the pressure tester, chances are real good either a head gasket is leaking, or a cylinder head is cracked. In either case, you'll have a cloud of white steam from the tail pipe when the engine is running, and the cooling system pressure will increase dramatically, to the point of blowing a hose, radiator, heater core etc.

Keep in mind that P0300 is almost always caused by a fault in secondary ignition. In some cases low fuel pressure will also cause it.