What is the "pressure gradient" within the richards equation
The pressure as measured at a point z+dz (dz infinitesimally small) and z.

At the surface, pressure is the atmospheric pressure. Just below the surface, let's say at 1 cm depth, pressure is atmospheric pressure + whatever weight of the water column (taking into account saturation and so on).

If the pressure gradient is in equilibrium with the gravity, you will have no flow (water in a glass do not mix, although at the top of the glass the pressure is lower than at the bottom, the water at the bottom feels a little push upwards equal and contrary to the push downward it receives from the weight of the water just above)

Think about this: you are at the bottom of a water reservoir (bottom not sealed). Pressure is the atmospheric pressure + depth of the reservoir. This is enough to induce a flow of water outwards from the reservoir, seeping below/through the dam.

Figure is not my own work, it is from a publication of Olsen and Stephens (2016) that can be found at this URL https://www.researchgate.net/publication/319206472_RELEARNING_HOW_TO_LOOK_AT_PIEZOMETRIC_DATA_FOR_SEEPAGE_EVALUATION