First-principles density functional theory–generalized gradient approximation methods are used to calculate the energetics of Ga in Ga-stabilized δ phase Pu, Pu2O3 and PuO2, in order to elucidate the relative stability of Ga in the Pu oxide layers. The Hubbard parameter U is used to describe the strongly correlated electron behavior of Pu 5f electrons. Three incorporation sites for Ga, i.e., interstitial site, Pu and O vacancies, are considered. The results indicate that the energetics of Ga significantly depend on the inherent properties of the host materials and the incorporation sites in them. Ga incorporation into interstitial sites and O vacancies of both Pu2O3 and PuO2 are energetically unfavorable. Ga incorporation into Pu vacancy of PuO2 is the most energetically favorable, followed by Ga incorporation into Pu vacancies of Pu2O3 and Pu. However, this does not mean that there would be the highest concentration of Ga in PuO2 among these three Ga-stabilized Pu substances because it is most difficult for Pu vacancy formation in PuO2. The distribution of Ga in the Pu oxide layers is proposed to be strongly dependent on the distribution and the concentration of Pu vacancy, as a result, Ga concentration likely decrease with the transitions of Pu→Pu2O3→PuO2.