zeroGradient pressure boundary condition on walls
 

I have a question regarding the zero Gradient boundary condition in OpenFOAM. Currently I simulate a supersonic flow passing over an obstacle using a K-omega SST model. I assumed the wall condition is adiabatic and no-slip and zero Gradient pressure (based on the tutorial case). My curiosity is, how the wall could reflect the pressure waves when the zero Gradient pressure boundary condition is applied? Could someone please explain me to understand.