  The Gaia SpectroPhotometric Standard Stars (SPSS) survey started in 2006, it
was awarded almost 450 observing nights, and accumulated almost 100,000 raw
data frames, with both photometric and spectroscopic observations. Such large
observational effort requires careful, homogeneous, and automated data
reduction and quality control procedures. In this paper, we quantitatively
evaluate instrumental effects that might have a significant (i.e.,$\geq$1%)
impact on the Gaia SPSS flux calibration. The measurements involve six
different instruments, monitored over the eight years of observations dedicated
to the Gaia flux standards campaigns: DOLORES@TNG in La Palma, EFOSC2@NTT and
ROSS@REM in La Silla<EMAIL_ADDRESS>in Calar Alto, BFOSC@Cassini in Loiano, and
<EMAIL_ADDRESS>in San Pedro Martir. We examine and quantitatively evaluate the
following effects: CCD linearity and shutter times, calibration frames
stability, lamp flexures, second order contamination, light polarization, and
fringing. We present methods to correct for the relevant effects, which can be
applied to a wide range of observational projects at similar instruments.
