Electrical properties and deep electron and hole trap spectra were measured for undoped n-GaN cut from a thick boule grown by hydride vapor phase epitaxy (HVPE). The material is characterized by a very low concentration of residual donors (1013–1014cm-3) in the first 30 μm near the growth surface. The bulk electrical properties were similar to those of standard high quality undoped bulk HVPE n-GaN, with a net donor concentration of ∼1016cm-3 and mobility ∼1000 cm2/V s. The strong decrease of electron concentration in the surface region of the high resistivity GaN was caused by the compensation of shallow residual donors by a high density (∼6 × 1015cm-3) of hole traps with activation energy of 0.2 eV, confined to the compensated region. In addition, other hole traps H5 with activation energy 1.2 eV and concentration 5 × 1015cm-3 were present. These latter traps had similar concentrations in both the high resistivity and standard conducting HVPE GaN. Radiation detectors prepared on the high resistivity material showed charge collection efficiency (CCE) close to 100% for spectrometry of α-particles with energy up to 5.1 MeV. The CCE dependence on voltage indicated a strong trapping of holes in the active region of detectors by the H5 hole traps.