how sure are you that it's the head gasket that's leaking ??
have you put the gasket the right way round , what i mean is are the gasket thickness holes towards the rear of the engine and on the pump side ?
and also there are two locating lugs on top of the block surface that must be fully inserted in the holes .
there is also a black plastic engine breather that bolts to your rocker cover and these can leak oil , they have a rubber o ring on the bit that slides in the rocker cover and are pennys to replace .
also , did you make sure that the head bolt holes were clean and free from crap ? such as old gasket bits , as this could stop the bolts tightening fully.