Is there a way to get the gpm if you know your psi?

 

Yes, with qualifications.

There are 2 types of pressure. Static and "operating" (I forgot the real term). The static pressure is when everything is at rest and no water is moving, this will be your max pressure. Once water starts to move, pressure drops to a steady level, (for the most part) and this would be the operating pressure. You will need to know this pressure and the size of pipe you are measuring.
Years ago I measured fire flows at fire hydrants using pressure drop and how far it took for the water stream from a hose connection to fall 12". Forgot the formula, (getting old), but should be fairly easy to find on internet with an engineering site.

Have fun on the web doing a quick search.

Found this site in just a few seconds, you might give it a look...

http://www.irrigationtutorials.com/sprinkler03.htm

 

I'm not sure what you are ultimately trying to do but Static pressure is really not a very useful number if you talk about GPM in the same sentence. What you are looking for can be calculated but the easier way is to get a garden hose, a five gallon bucket, and a watch. Time how long it takes to fill the bucket and calculate GPM from there. While the water is running, note the pressure.

 

Residual pressure is the term you was thinking of.

 

Thanks to all for the responces, I was going to try the bucket but then notuboo put the link for Irrigation Tutorials great site.
Thanks again.

 

Yep. Jess Stryker's site is excellent, and also please check Ron (Pumpman) Peeks' site at www.peekspump.com
They are absolutely the best tutorials that I have seen on the subject.
Good luck!
Mike