Engineering microstructural features such as grain size in ceramic materials have strong effect on the various properties of materials. In this manuscript, we demonstrate the effect of grain size on electrical transport in polycrystalline gallium ferrite (GaFeO3 or GFO) samples prepared by solid-state-reaction method. We varied the grain size of the polycrystalline GFO samples by changing the duration of sintering, increasing from 9 μm to 21 μm with the increase in sintering time from 12 h to 256 h at 1350 °C. The samples remain phase-pure without any secondary phase formation upon changing sintering duration. Leakage current density of the samples reduces by nearly four orders of magnitude as the grain size increases from 9 μm to 21 μm. Time and temperature dependent impedance measurements revealed that while the grain boundaries in GFO samples were more insulating than the grains with higher activation energy of 0.65–0.95 eV in comparison to 0.30–0.47 eV for the grains, grain resistivity increases with increase in the sintering duration. Latter is attributed to increasing grain conduction activation energy, caused by migration of oxygen vacancies toward grain boundaries as the sintering time increases resulting in a concomitant decrease in grain boundary conduction activation energy. The study suggests towards a subtle balance between grain and grain boundary conductivities which eventually drive the overall electrical leakage of GFO ceramics. The study also highlights the role of microstructural engineering in tailoring the electrical properties of magnetoelectric GFO ceramics without resorting to atomic substitution.