  The second data release (DR2) of Gaia provides mean photometry in three bands
for $\sim$1.4 billion sources, but light curves and variability properties are
available for only $\sim$0.5 million of them. Here, we provide a census of
large-amplitude variables with amplitudes larger than $\sim$0.2 mag in the $G$
band for objects with mean brightnesses between 5.5 and 19 mag. To achieve
this, we rely on variability amplitude proxies in $G$, $G_{BP}$ and $G_{RP}$
computed from the uncertainties on the magnitudes published in DR2. We then
apply successive filters to identify two subsets containing respectively
sources with reliable mean $G_{BP}$ and $G_{RP}$ (for studies using colours)
and sources having compatible amplitude proxies in $G$, $G_{BP}$ and $G_{RP}$
(for multi-band variability studies). The full catalogue gathers $23\,315\,874$
large-amplitude variable candidates, and the two subsets with increased levels
of purity contain respectively $1\,148\,861$ and $618\,966$ sources. A
multi-band variability analysis of the catalogue shows that different types of
variable stars can be globally categorized in four groups according to their
colour and blue-to-red amplitude ratios as determined from the $G$, $G_{BP}$
and $G_{RP}$ amplitude proxies. The catalogue constitutes the first census of
Gaia large-amplitude variable candidates, extracted from the public DR2
archive. The overview presented here illustrates the added-value of the mission
for multi-band variability studies even at this stage when epoch photometry is
not yet available for all sources. (Abridged abstract)
