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The thermal conductivities of (100) γ-Ga2O3 films deposited on (100) MgAl2O4 substrates with various thicknesses were measured using frequency-domain thermoreflectance. The measured thermal conductivities of γ-Ga2O3 films are lower than the thermal conductivities of (2¯ 01) β-Ga2O3 films of comparable thickness, which suggests that γ-phase inclusions in the doped or alloyed β-phase may affect its thermal conductivity. The thermal conductivity of γ-Ga2O3 increases from 2.3−0.5+0.9 to 3.5±0.7 W/m K for films with thicknesses of 75–404 nm, which demonstrates a prominent size effect on thermal conductivity. The thermal conductivity of γ-Ga2O3 also shows a slight increase as temperature increases from 293 to 400 K. This increase in thermal conductivity occurs when defect and boundary scattering suppress signatures of temperature-dependent Umklapp scattering. γ-Ga2O3 has a cation-defective spinel structure with at least two gallium vacancies in every unit cell, which are the likely source of defect scattering.