The effect of gamma radiation dose (∼1 MGy) on the performance of negative-index fiber Bragg grating (FBG) formed by prolong UV laser exposure (type-IIa grating) and prolong thermal exposure (thermally regenerated grating) is presented for the first time. It is observed that for both negative-index gratings, the radiation-induced shift in Bragg wavelength and temperature sensitivity were less than 37 pm and 0.06 pm∕°C, respectively. The performance of negative-index grating is also compared with that of positive-index grating (type-I) under the same gamma irradiation conditions. It is observed that variation in Bragg wavelength, reflection intensity, and temperature sensitivity was lower in negative-index gratings compared to that of positive-index grating. The effect of radiation is analyzed in terms of “radiation-optic coefficient” for all FBGs. The different behavior of the FBG is linked with a different mechanism responsible for the formation of negative- and positive-index FBGs.