[en] A system of computer programs intended for solution of the plane one-dimensional photon transport equation in the case of two adjacent media is described, and user's guides for the programs are given. One medium represents a natural ground with uniformly distributed potassium, uranium, and thorium gamma-ray emitters. The other medium is usually air with no radioactive contaminants. The solution method is the double-P₁ approximation with logarithmic energy spacing. The complete data-processing system GB contains the transport-theory code GAMP1, the code GFX for computation of scalar flux and dose rate, and a number of auxiliary programs and data files. (author)