In this paper we examine the effect of Ga doping on the structure and conductivity of the high Li ion content garnet-related system, La₃Zr₂Li₇O₁₂. Without Ga doping, La₃Zr₂Li₇O₁₂ is tetragonal and has low Li ion conductivity. The introduction of Ga leads to a change to a cubic unit cell, and a large enhancement in the conductivity. Prior structural studies of La₃Zr₂Li₇O₁₂ have shown the presence of both tetrahedral and distorted octahedral sites for Li, and the low conductivity can be explained by the ordered nature of the Li distribution. The present structural study of La₃Zr₂Ga_0.5Li_5.5O₁₂ shows that Ga substitutes onto the tetrahedral site. Despite the presence of non-mobile Ga³⁺ on the Li sites, the conductivity is enhanced as a result of the introduction of vacancies in the Li sites, and consequent disorder on the Li sublattice. Further work has suggested that over time in air, there is some H⁺/Li⁺ exchange, and consequently some variation in the conductivity.