They’re both as equally important as each other and are worth understanding and taking into consideration. The PSI will indicate how much you’ll be able to clean off a surface whereas the GPM will tell you how much rinse you’ll get from your machine. For a farmer, the GPM will be more critical as their principal concern is fast rinsing and flushing. Someone tasked with cleaning up dirt and paint from concrete and paving though is likely to be more interested in the PSI measurement.\r