The performance of III-Nitride high power, high frequency transistors and laser diodes is intimately connected with the ability to dissipate heat from the junction to the substrate. The thermal conductivity was characterized by the three omega method for undoped and doped gallium nitride bulk substrates grown by HVPE from room temperature to 450 K. The thickness of the samples varied from thin film epilayers on sapphire to 2 millimeter thick free standing samples Dislocation density of the substrates was measured by imaging cathodoluminescence, SIMS was used to measure impurity levels of oxygen, hydrogen, silicon, and iron, while carrier concentrations and resistivity were determined from electrical measurements and EPR. A semi-insulating, 2 mm thick iron doped sample had the highest thermal conductivity of 230W/K-m at room temperature. Undoped samples had comparable, but lower thermal conductivities throughout the temperature range from 300-450 K. By comparing these results with previously reported experimental results including those on MOCVD grown GaN free of grain boundaries, we establish an empirical relationship in a compact formula that relates the thermal conductivity of GaN and the dislocation density with three different regimes of low, intermediate, and high dislocation densities. In the high dislocation regime, the thermal conductivity improves significantly with reduction of dislocation densities. As material quality continues to improve it remains to be seen if in the low dislocation density regime, thermal conductivities will approach 300 W/K-m or plateau out near 250 W/K-m. As point defects start to limit the thermal conductivity when dislocation density becomes very low, gallium vacancies are expected to play an increasing role. Iron is postulated to substitute on the gallium site. The indication from this study is that iron doping at concentration of 1018 cm-3 is not limiting the thermal conductivity in the 300-450 K range.