  In this paper we study G-surfaces, a rather unknown surface class originally
defined by Calapso, and show that the coordinate surfaces of a Guichard net are
G-surfaces. Based on this observation, we present distinguished Combescure
transformations that provide a duality for Guichard nets. Another class of
special Combescure transformations is then used to construct a B\"acklund-type
transformation for Guichard nets. In this realm a permutability theorem for the
dual systems is proven.
