What do we mean exactly mean by Zero Gradient?
OpenFOAM-version-7 Simulating-Hagen-Poiseuille-flow-through-a-pipe-in-OpenFOAM 03-04 min 10-20 sec
Zero gradient (ZG) condition at the boundary implies that as you reach that boundary, the parameter for which you have imposed ZG doesn't change normal to the boundary. In CFD codes, this boundary condition is usually imposed using 'ghost' cells, where a dummy cell is considered adjacent to the boundary and outside the domain and the value of those cells are imposed to be the same as the ones at the adjacent cells at the boundary.
A better pictorial representation is given in the link below.