The non-critical IUPAC compilation of stability constants (SC-database) is not longer available. You will find IUPAC data for Ge in the file: germanium.TXT.
A critical study of equilibrium constants for some inorganic l.m.m. ligands with gallium can be found at: S.A. Wood, I.M. Samson (2006) The aqueous geochemistry of gallium, germanium, indium and scandium. Ore Geology Reviews 28, 57–102.