On my washer (2000 psi, 2gpm) I could not get gravity to feed it the pressure washer enough. I even put my tank on a stand. I bought a 12v pump and battery and hooked it up, that way I have enough gpm to the pressure washer so it doesn't burn out, and also can fill a bucket with water if I need to.
The biggest thing you want to make sure of is that you get enough gpm to your washer (at least for what it is rated). You do not have to worry about how much pressure you have to the pressure washer. Hope that helps, any more questions, let me know.