2007 Greyhawk 30g which lowpoint drain is water heater?

Use the drain on the outside of the water heater. It is either a plastic plug or an anode rod, depending on the manufacturer of the heater.

One way to probably find out would be to turn off the water supply, open some faucets, and try the drains in sequence. The one that drains for a comparatively long time would presumably be draining the water heater.

I'd still double-check with the heater drain plug afterwards to make sure it actually drains via the low point drains.

I don't have a greyhawk but I can drain my water heater thru one of the low point drains (cold water system drain).
My WH doesn't have any check valves so it will flow back thru cold inlet line and out low point drain .....just have to open kitchen faucet hotside (closest faucet) to allow air back into system.

But removing drain plug is just as easy

I doubt there is a low point drain for the water heater.
To drain the water heater you need to remove the anode rod/plug on the heater itself.