  The cubic-kilometre neutrino telescope, which consists of large-scale
3D-arrays of photomultiplier tubes (PMTs) currently under construction on the
Mediterranean seabed, relies on accurate calibration procedures in order to
answer its science goals. These proceedings present the gain calibration method
used in KM3NeT, which is based on highly compressed PMT hit information. In
particular, it is shown that the PMT gains can be tuned to within 2% of the
nominal value, based on the measured single photoelectron time-over-threshold
distribution of each PMT.
