The quality of n − GaN epilayers grown by Metalorganic Vapor-Phase Epitaxy (MOVPE) on either ammonothermal or Halide Vapor-Phase Epitaxy (HVPE) substrates were analyzed by Deep-Level Transient Spectroscopy (DLTS) on identical vertical Schottky diodes between 220 K and 450 K. The acceptor trap E3 ( E C − E T ≃ 0 . 58 eV ) commonly observed on GaN is identified with a concentration lower by one to two orders of magnitude for epilayers grown on ammonothermal substrates. Alongside E3, a second defect, Ex ( E C − E T ≃ 0 . 83 − 0 . 88 eV ) is observed. This acceptor defect associated to interstitial nitrogen is only observed for epilayers on HVPE substrates. Varying the filling time pulses indicates that the trap Ex can align with extended defects. The lower E3 concentration and the absence of Ex highlight the higher suitability of ammonothermal substrates for device-grade epilayer growth.