Contact the chiller manufacturer and request a flow curve for your chiller. You will need to determine pressure drop across the evaporator then plot it on the flow curve. This should tell you the gpm through the chiller evaporator. Pump curves will also work if you don't have pressure gauges or ports at the evaporator. Try to use the same gauge for both pressure readings and make sure the gauge is good. Most flow curves will read in FT of Water or Ft Head. You can convert your pressure drop (or delta P) to Ft by multiplying times 2.31.