  A characteristic of every inorganic scintillator crystal is its light yield,
i.e., the amount of emitted scintillation photons per unit of energy deposited
in the crystal. Light yield is known to be usually non-linear with energy,
which impacts the spectroscopic properties of the scintillator. Cerium-doped
gadolinium-aluminium-gallium garnet (GAGG:Ce) is a recently developed
scintillator with several interesting properties, which make it very promising
for space-based gamma-ray detectors, such as in the HERMES nanosatellite
mission. In this paper we report an accurate measurement of the GAGG:Ce
non-linearity in the 20-662 keV gamma-ray energy interval, using a setup
composed of three samples of GAGG:Ce crystals read out by Silicon Drift
Detectors (SDDs).
