In addition to the concepts of gage and absolute pressure, for simulation with incompressible flow in general, keep in mind that the solver sees pressure not as a thermodynamic quantity but a mechanical quantity which enforces continuity of flow, or mass balance. Therefore the solver is only interested in the variation of pressure within the calculation domain.
As an example, suppose you have a pipe flow problem where you apply the velocity or flow rate at the inlet and a zero gage pressure at the outlet. Then, the inlet average pressure, which will be calculated by the solver, will be equal to the pressure drop inside the pipe because the pressure solved at the inlet is the so-called gage pressure and so the pressure drop is simply the inlet pressure minus the exit pressure which was set to zero. Let use assume that this pressure drop is equal to X units.
Now, take the same pipe with the previously applied inlet flow rate, but set the exit gage pressure condition to 1000 units. When the solver finishes with a converged solution, you will see that the velocity field is almost exactly the same as in the previous analysis, and that the average pressure calculated by the solver at the inlet is very near to 1000 + X units. Therefore, the pressure drop in this analysis is 1000 + X - 1000 = X units, exactly the same as in the previous run, which is to be expected.
In summary, when running an incompressible flow analysis with constant density, it is the change is pressure inside the device and not the values applied on its boundaries that is of importance.
Hope this helps.
