Both the gates are internally connected and they are grounded yielding zero gate source voltage (VGS =0). The word gate is used because the potential applied between gate and source controls the channel width and hence the current.
As with all PN junctions, a depletion region is formed on the two sides of the reverse biased PN junction. The current carriers have diffused across the junction, leaving only uncovered positive ions on the n side and negative ions on the p side. The depletion region width increases with the magnitude of reverse bias. The conductivity of this channel is normally zero because of the unavailability of current carriers.
The potential at any point along the channel depends on the distance of that point from the drain, points close to the drain are at a higher positive potential, relative to ground, then points close to the source. Both depletion regions are therefore subject to greater reverse voltage near the drain. Therefore the depletion region width increases as we move towards drain. The flow of electrons from source to drain is now restricted to the narrow channel between the no conducting depletion regions. The width of this channel determines the resistance between drain and source.