Depends on what you are growing and how you water...What is better to water your plants with--- rainwater or gray water?
Gray water is often full of phosphates, and that is a fertilizer.
Gray water is OK to irrigate with, but DO NOT SHOWER GRAY WATER ON THE PLANTS!
Any bacteria living in the 'Gray Water' will be directly applied to the leaves and edible parts and that would be a bad thing.
"Gray Water" delivered to the plant bases, or even sub-surface by low pressure 'Trickler' hoses is a great way to water!
Rain water is always good, although it carries no fertilizer value.
Since rain water is usually clean, showing or irrigation is fine.
Remember, most of the water you 'Shower' the garden with will evaporate unless you have ground cover like straw on the ground...
Better to trickle water or irrigate than the spray or 'Shower' the plants when you can.