GPM stands for Gallons Per Minute. It tells you how much water will be moved through the pump and delivered to the point of cleaning. It's much more important than the PSI specifications for your pressure washers.
Time is Money
After you reach the correct PSI for your cleaning job, the GPM rating will tell you how fast a particular pressure washer will get the job done. The higher the figure, the less time a job will take – and the lower the manpower costs for that job. Of course, higher GPM (and PSI) are only delivered by bigger motors and pumps, so there is a tradeoff.
Another factor to consider is what happens to the water which will likely contain soaps/chemicals and whatever dirt/grease you are removing.
Remote Pressure Washing
If your pressure washer is to be used at a job site or other remote location where you’ll need to bring your water with you, the GPM rating can become an even more important factor in your decision on which pressure washer is best for you.