Theeffects　of　gamma　rays　on　two　kinds　of　GaN　high-electron-mobility　transistors　(HEMTs)　have　been　investigated　in　this　paper.　We　have　identified　a　gate　degradation　using　a　combination　of　optical　and　electrical　measurements.　We　have　demonstrated　that　the　channel　current　under　the　degradation　position　is　outside　of　the　gate＇s　control.　This　degradation　prevents　the　gate　from　fully　pinching　off　the　channel,　creating　a　current　concentration　region　when　applying　a　reverse　gate　voltage.　An　integrated　analysis,　including　I-V　characterization,　the　emission　microscope　(EMMI)　technique,　and　temperature-dependent　measurements,　was　applied　to　study　its　mechanism.　It　is　attributed　to　an　irradiation-induced　degradation　in　the　Schottky　contact.