Question
I'm currently designing an irrigation system for my home and have a question about the interaction between PSI and GPM.

If I buy a single head that is rated at 40 GPM / 2.0 GPM and add another head with the same specs, will they both still produce 2.0 GPM?  Can I continue to add heads until my maximum GPM is reached (10 GPM at the spigot) and expect coverage as stated, or at least within 5' of rated radial coverage?

Or, does GPM output of a head increase based upon the number of heads in the zone?

Answer
A sprinkler nozzle flow rate is based on how much water will go through a specified hole size at so much PSI. So, a 1, 2 GPM will flow 2 GPM. 4, 2 GPM nozzles will flow 8 GPM. An individual  sprinklers flow rate will not change with additional heads. You just add them together for your total zone size.
The complication to irrigation design is pipe size. As you increase water flow you also increase the pressure loss on the pipe. Too small of pipe and you end up with 40PSI at the first head and it diminishes with each head down the pipe. That will affect GPM of each head. Rather than go through a complicated friction loss explanation. I'd recommend 1" pipe for you system. At 10GPM you'll have no problems. And you only have a few types of fittings you need to purchase.
Hope this helps.

Matt