Generate electricity, water wheel
One simple sanity check is that you will get less power out (due to imperfect efficiency) than the power that would be required to have a pump circulate water from the outflow to the input. If the head pressure is low, you'll find that a very low power pump will pump 1 GPM up a few feet - so your power out will be lower than that. If your 1 GPM is coming in under pressure from a source a few hundred feet above the wheel, you might have some serious power.