In this work, a comparison study of four different freestanding GaN substrates is performed by Deep-Level Transient Spectroscopy (DLTS) on Schottky diodes. Three defects are clearly identified, E1 (ET = 0.08-0.14 eV), E2 (ET = 0.55-0.60 eV) and E3 (ET = 0.86-0.91 eV). Comparing the concentration for the different substrates reveals the absence of E3 trap and a lower concentration of E2 trap for the epilayer on the substrate grown within GaN4AP consortium by the ammonothermal technique.