Pressure only comes when the flow is restricted.
Max flow comes when there is no restriction.
GPM is strictly a function of displacement X frequency (RPM)
The specifications are at odds.
Think of it, What is the "pressure" of the water as it flows out of your kitchen tap?
You might have an idea that the pressure drops when flow is unrestriced if you have ever taken a shower and had the toilet flushed ;-) The flow restriction in this example is the shower head, and water pipes.
In the same way, a tiny pump can develope large pressure values if the flow is very small. Dead end a positive displacement pump, and something breaks! That is why there are pressure relief valves built into most every pump.
Pressure and volume are difficult to relate. But when you do, WORK , i.e force through a distance, will become understandable, and with the added element of time, "Horsepower" becomes calculable.