It's the gradient of the potential i.e. Newtonian 'g-field' that sums to zero. Inside, the potential -GM/R is depressed uniformly, where R is the effective radius of the shell. Note that in GR, R is defined in terms of proper surface area or proper circumference. Something needful when dealing with a dreaded black hole. For which the usual definition for R as proper length of a radius vector from centre of a sphere to it's periphery, breaks down completely.