how the heck do ya get a gravely to turn over?

You say that you removed the head and everything was fine? But you can't turn it over with a starting rope? Sounds like the piston may be frozen in the cylinder.

Spray some engine oil through the spark plug hole. Let it soak for a while. Can you get a pipe wrench (or large vice grips on) on anything that connects to the crankshaft? Maybe the start pulley base or the drive pulley on the other side of the engine? Try turning it with a pipe wrench (gently, don't want to damage anything). If you can get a little bit of movement, try alternating direction to get it moving - back and forth.

The above may sound a bit brutal, but my chainsaw seized up (3 times) on me while cutting big/hard oak trees into firewood several years ago. I worked it loose each time out and it's still running good today.

You probably have excessive oil remaining inside the cylinder, which was approaching hydraulic lock conditions. To remove excess oil, pull the spark plug and use the starter rope to spin it over as fast as possible and blow out the oil.

Take care that the rope doesn't hit you in the eye when pulling!!