I looked at the ball valve diagram and you need to apply voltage in one direction to open and apply voltage the other direction to close the valve. I suggest that you try operating the valve with straight 12V first then try switching the voltage in with relays.
Also, to make this valve work correctly you will need to use two relays, with the relay commons wired to the valve and the relay NO & NC connections alternately wired to 12VDC and ground.
I'm not doing gas because I don't have natural gas at the house & propane is too expensive, but if I were I don't believe I would be using a solenoid that required a voltage to close because it's not fail safe. I'd use something like one of these that requires voltage to stay open, so that if I loose power the solenoid snaps shut.