GPM stands for gallons per minute while PSI stands for pounds per square inch. The GPM is a measurement of how much water comes out of a pressure washer. PSI is a measurement of how strong the force of water is coming out. This means that the PSI is what provides the power to penetrate deeper into the pores of the surface and gallons per minute is what allows you to spread the work over a larger area. A machine with a high PSI, but low GPM will result in a machine that can clean small areas well but is insufficient for cleaning larger projects. On the other end would be a machine with high gallons per minute, but low PSI. This machine would be great for rinsing dirt away, but would not be great for removing tough stains. The effectiveness of a pressure washer depends on how the PSI and GPM work in tandem. Generally, having a pressure washer with high figures for both GPM and PSI means a faster and easier time in cleaning. The key thing to remember when it comes to power washer GPM versus PSI is that these have to work together. This means that both should match well for optimal cleaning results.