Gallons per minute, abbreviated GPM, measures a volume of liquid moving each minute. It is a flow rate, not a measure of pressure.
What this means at your property
A pump may advertise a maximum flow rate under particular test conditions. The actual rate can change with the lift, hose arrangement and equipment setup. A number on a specification sheet is therefore not a promise of how quickly a room will be emptied.
What should be compared when choosing equipment?
Compare performance under the intended operating conditions and identify which gallon unit is used. Ask the contractor to distinguish water extraction capacity from the later work of drying materials.