  The magnetic chemically peculiar (mCP) stars of the upper main sequence are
perfectly suited to studying the effects of rotation, diffusion, mass-loss,
accretion, and pulsation in the presence of an organized stellar magnetic
field. Therefore, many important models can only be tested with this star
group. In this case study we investigate the possibility of detecting the
characteristic 520 nm flux depression of mCP stars using low-resolution BP/RP
spectra of the Gaia mission. This would enable us to effectively search for
these objects in the ever-increasing database. We employed the tool of Delta a
photometry to trace the 520 nm flux depression for 1240 known mCP and 387
normal-type objects including binaries. To this end, we folded the filter
curves with the BP/RP spectra and generated the well-established color-color
diagram. It is clearly possible to distinguish mCP stars from normal-type
objects. The detection rate is almost 95\% for B- and A-type objects. It then
drops for cooler-type stars, which is in line with models of the 520 nm flux
depression. The BP/RP spectra are clearly qualified to efficiently search for
and detect mCP stars.
