Mark,
There is not a strait-forward answer to this.
Firstly the GPM used is dependant on the relationship between potential pump output, pressure and jetting.
The quoted GPM rating of the pump is normally its' maximum open-flow output.
The actual GPM that exits the wand will depend upon the jetting and the pressure setting - increase either and there is an increase in water output.
having a pump that has the potential to output more gallons per minute than will ever be used is generally better. To illustrate, a 100psi Flowjet has a relatively low output potential. Resultantly, when the wand trigger is pulled the pressure will drop perhaps as low as 80psi, as the pump cannot supply sufficient water to maintain the full pressure. With a more capable pump however, the pressure will remain close to the closed system pressure.
As to the 'ideal' GPM usage, this itself is dependant on several factors such as the flushing demands of the job, stroke speed etc. This is something that most cleaners adjust for themselves, not normally by changing jets for each job but by altering pressure and stroke speeds.