as a first thought if you have a spare channel on your transmitter use it to drive a servo to close a shut off valve it could also function as a valve to prevent accidental gas leakage when connecting everything up. Trying to do it auto is fraught with danger since even if your out of water your still going to have pressure. if on the other hand your wanting to stop the gas if your pressure falls to low why?
"that's not a bug its just an undocumented creature."
Sir Terence David John "Terry" Pratchett, OBE (28 April 1948 - 12 March 2015)