If you trust your P test kits, and your levels of P are not measurable, you may not need to run GFO. For one, GFO is expensive, and more importantly, your corals needs some P to grow. I would consider taking a baseline P measurement, take the GFO offline and test your P a day later. If you are not measuring any P, I'd take the GFO offline until you start to measure P, and moderate its use to get your P in the range of 0.02-0.04ppm.
What I'm suggesting is essentially what I've read somewhere on the RC Reef Chemistry forums when I was researching running GFO on my system. The range 0.02-0.04 is what many of the best looking reefs are dialed into. My SPS growth, PE and coloration seem to be best in that range.
Some folks also recommend keeping a close eye on your Alk when running GFO, as some have experience with Alk depleting more rapidly when running GFO.
