I assume that since you only mention head gasket as in singular, you are working on a 4 cylinder motor. The only way I know to get coolant in the oil is to have an internal leak. This is not just caused by cracks in the head or block, or by a blown head gasket, but by a warped cylinder head. Since your car overheated and shut down I assume that it got hot enough to do just that. Have the cylinder head checked at a machine shop to see if the surface is true. If not, it can be trued up. Get a new gasket and properly torque it down onto the block. That should fix the problem.