Insufficient information. Pressure, Temperature and Composition will give you state information, but not mass flow rate. You may be able to approximate it by assuming another piece of information (maximum velocity in the pipe).
You need a second pressure gauge, ideally located some distance down the line. Straight pipe is the easiest to deal with, but a few elbows aren't too hard to figure. If you can get the pressure drop in say, 100 feet of straight pipe with no reducers, branches or valves, that's best.