Absorption spectra of ultraviolet-irradiated single crystals of gadolinium gallium garnet are investigated. Colour centre bands are observed at 440 and 320 nm. The absorption can be enhanced by doping with Tb, and suppressed by doping with Eu. Annealing treatments under a low partial oxygen pressure cause a decrease of the absorption after irradiation. A similar effect is found after heating the garnet covered with a silicon layer. An increased colorability is obtained after annealing in oxygen. Glow curves and thermally stimulated electrical conductivity show the recombination of charge carriers at rare earth sites. From these observations it is concluded that the colour centre bands are due to trapped electrons, possibly at interstitial gallium sites.