A gallium nitride crystal with a polyhedron shape having exposed {10-10} m-planes and an exposed (000-1) N-polar c-plane, wherein a surface area of the exposed (000-1) N-polar c-plane is more than 10 mm2 and a total surface area of the exposed {10-10} m-planes is larger than half of the surface area of (000-1) N-polar c-plane. The GaN bulk crystals were grown by an ammonothermal method with a higher temperature and temperature difference than is used conventionally, using a high-pressure vessel with an upper region and a lower region. The temperature of the lower region is at or above 550° C., the temperature of the upper region is set at or above 500° C., and the temperature difference between the lower and upper regions is maintained at or above 30° C. GaN seed crystals having a longest dimension along the c-axis and exposed large area m-planes are used.