No Compression

Often when an engine sits, the valves stick. A used small block I picked up was like that. I tapped the top of each valve with a hammer until it popped up. You must remove the rocker arm assembly first (easy) and first spray each stem with WD-40 or similar. Another guy seconded my comment that it is extremely unlikely that a bad head gasket could take out 5 cylinders, so spend more time diagnosing before you pull the head and probably waste a lot of time.