A 3-dimensional position-sensitive CdZnTe gamma-ray detector employs a pixellated anode (11 ?? 11 pixels) and a planar cathode on a 15 ?? 15 ?? 10 mm3 or 20 ?? 20 ?? 15 mm3 CdZnTe single crystal. The signals from the 121 pixels and the cathode read out by Application Specific Integrated Circuits (ASIC) are used to reconstruct the energy depositions and 3-D coordinates of multiple gamma-ray interactions inside the detector. The best energy resolution of a given detector can be achieved by using the 3-D position sensitivity to correct the signal variations due to charge trapping and material non-uniformity. We have demonstrated excellent energy resolution of better than 1% FWHM at 662 keV in a dozen CdZnTe detectors ranging from 2.25 cm3 to 6 cm3 in size. In this study, the energy resolution of four 6 cm3 3-D position sensitive CdZnTe detectors at gamma-ray energies higher than 1 MeV (up to 2.6 MeV of 228Th) are reported. Various challenges in events calibration/reconstruction at high gamma-ray energies and possible solutions are discussed.