When the head was off to replace gasket... was the head and block inspected for cracks?
You could try a 'leakdown' test.... replace radiator-cap with special cap to allow pumping pressure into system... if it bleeds-down, there is a problem somewhere.
Also, when cranking the engine (do not allow it to start), it is often audable if one of the cylinders does not have compression. There will be kinda 'skip' each time the weak hole tries to make compression.
You may end up pulling the head again... and IMMEDEATLY inspect for water in one of the holes.