We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×10 19 cm −3. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl 4 as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×10 6 cm −2. The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm 2 V −1 s −1 for n=3.3×10 18 and 3.35×10 19 cm −3, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm −1 K −1. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10 −6 K −1 in the measured doping concentration range.