Re: How can I calculate water flow (gpm) by using a PSI guages?

You can calculate the flow velocity and, from it, the flow rate, using a
pitot-static tube, or some other means of measuring stagnation (total)
versus static pressure. Once you have the total pressure, P0,  and the
static pressure, P, (or simply the diference between them), and you know
the density of the fluid, rho, you can calculate the velocity using the
following relation,

        P0 - P = (rho)*V^2

Once you have calculated the velocity, and knowing the cross-sectional
area of the passage, A, you can calculate the volumetric flow rate, Q, as

        Q = V*A

or the mass flow rate as

        m' = (rho)*V*A

Any undergraduate text in fluid mechanics (like Fox & McDonald or
White) will describe these relationships in much greater detail.
Hope this helps.

Frederick J. Carranti, P.E.