The Gerda experiment at Lngs of INFN is equipped with an active muon veto. The main part of the system is a water Cherenkov veto with 66 PMTs in the water tank surrounding the Gerda cryostat. The muon flux recorded by this veto shows a seasonal modulation. Two causes have been identified: ( i ) secondary muons from the Cngs neutrino beam (2.2%) and ( ii ) a temperature modulation of the atmosphere (1.4%). A mean cosmic muon rate of Iμ0=(3.477±0.002stat±0.067sys)×10−4 /(s · m 2 ) was found in good agreement with other experiments at Lngs . Combining the present result with those from previous experiments at Lngs the effective temperature coefficient αT,Lngs is determined to 0.93 ± 0.03. A fit of the temperature coefficients measured at various underground sites yields a kaon to pion ratio r K / π of 0.10 ± 0.03.