The effects of fast neutron (up to 1.75 × 1020 f.n. cm–2) and fast plus thermal neutron (up to 3.5 × 1020 f.t.n. cm–2) irradiation on the electrical properties and crystal lattice of GaN films have been studied. It is shown that high-dose neutron irradiation induces a transition of n-type samples into a semi-insulating state with resistivity of ~1010 Ω cm at room temperature, while p-type films undergo transition to a high-resistance state. The subsequent irradiation of the samples leads to a decrease of resistivity to ~105 Ω cm at the final neutron fluence. Raman spectra show appearance of the disorder-activated Raman scattering modes at 210, 300 and 670 cm−1 linked to defects in Ga and N sublattices. X-ray diffraction measurements reveal an expansion of the GaN c-lattice parameter up to a saturation level of 0.42% at the final fluence, while the a-lattice parameter remains nearly unchanged. The initial value of c restores at 1000 °C annealing temperature, with the main annealing stage at 150 °C–400 °C. The significant role of thermal neutrons (E ≤ 0.1 MeV) in the damage build-up in GaN is shown.