  We set out to study the atmosphere of WASP-80b, a warm inflated gas giant
with an equilibrium temperature of $\sim$800~K, using ground-based transmission
spectroscopy covering the spectral range from 520~to~910~nm. The observations
allow us to probe the existence and abundance of K and Na in WASP-80b's
atmosphere, existence of high-altitude clouds, and Rayleigh-scattering in the
blue end of the spectrum.
  We observed two spectroscopic time series of WASP-80b transits with the
OSIRIS spectrograph installed in the Gran Telescopio CANARIAS, and use the
observations to estimate the planet's transmission spectrum between 520~nm and
910~nm in 20~nm-wide passbands, and around the K~I and Na~I resonance doublets
in 6~nm-wide passbands. We model three previously published broadband datasets
consisting of 27 light curves jointly prior to the transmission spectroscopy
analysis in order to obtain improved prior estimates for the planet's orbital
parameters, average radius ratio, and stellar density.
  We recover a flat transmission spectrum with no evidence of Rayleigh
scattering or K~I or Na~I absorption, and obtain an improved system
characterisation as a by-product of the broadband- and GTC-dataset modelling.
The transmission spectra estimated separately from the two observing runs are
consistent with each other, as are the transmission spectra estimated using
either a parametric or nonparametric systematics models. The flat transmission
spectrum favours an atmosphere model with high-altitude clouds over cloud-free
models with stellar or sub-stellar metallicities.
