Insulator-metal transitions are well known in transition-metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here, we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO(1.2) at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga(2)O(3) within the metastable oxide matrix-in chemical terms by a disproportionation. This novel mechanism--an insulator-metal transition driven by a heterogeneous solid-state reaction--opens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.