As you lower the pressure on this water (you don't specify what its initial value) it will, at some point, start to boil . The lower you let the pressure go the more of it will flash off to steam. In other words you will have to specify a final pressure before you can get an answer in terms of steam quantity and temperature.
To get an accurate answer you should also say whether the water flow rate is in US gallons or Imperial gallons per minute. We don't even know where you live, which, if we did, might allow us to guess your preferred units!
