They’re equally important, and the combination is crucial. The PSI is what will provide the stripping action to lift and move dirt away. The GPM is more about the rinsing power that carries away the dirt you’ve lifted.
If you multiply the PSI by the GPM, you will get the cleaning unit (CU) which is an easier way to look at it for some customers.
A pressure washer with a 3600 PSI and a 2.4 GPM has a CU of 8640. The higher this number, the more powerful the pressure washer.