I was wondering the same thing.... Turns out you need to use precipitation rate, which is different from GPH (which is how emitters are labeled). This site provides a formula for precipitation rate on lawns, but I found this document which tells you how to calculate it for emitters
You could then calculate a precipitation rate for each of your groups of emitters, and then add them all together to get your total precipitation rate (which is what you'd put into the app).
Honestly, I think this calculation is silly. For emitters, it would make so much more sense if you could just type in the number of emitters and the flow rate as you suggest. But the app is clearly built for lawn sprinklers....