  The GCT is a dual-mirror Small-Sized-Telescope prototype proposed for the
Cherenkov Telescope Array. Calibration of the GCT's camera is primarily
achieved with LED-based flasher units capable of producing $\sim4$ ns FWHM
pulses of 400 nm light across a large dynamic range, from 0.1 up to 1000
photoelectrons. The flasher units are housed in the four corners of the
camera's focal plane and illuminate it via reflection from the secondary
mirror. These flasher units are adaptable to allow several calibration
scenarios to be accomplished: camera flat-fielding, linearity measurements (up
to and past saturation), and gain estimates from both single pe measurements
and from the photon statistics at various high illumination levels. In these
proceedings, the performance of the GCT flashers is described, together with
ongoing simulation work to quantify the efficiency of using muon rings as an
end-to-end calibration for the optical throughput of the GCT.
