Gross thickness is the whole interval from top to base. Net pay is what remains after rock that is too tight, too shaly, or too wet is taken out. A thin tight streak inside the interval is gross but not net. The thickness that enters a volumetric calculation is this net.
Cutoffs are choices. Two interpreters can report different net pay from the same logs by moving the porosity, shale, or saturation limit. Net reservoir, the porous rock whether or not it contains hydrocarbon, is larger than net pay. Quoting one as the other changes the volume.