  The Gaia Radial Velocity Spectrometer provides the unique opportunity of a
spectroscopic analysis of millions of stars at medium-resolution in the
near-infrared. This wavelength range includes the Ca II infrared triplet (IRT),
which is a good diagnostics of magnetic activity in the chromosphere of
late-type stars. Here we present the method devised for inferring the Gaia
stellar activity index together with its scientific validation. A sample of
well studied PMS stars is considered to identify the regime in which the Gaia
stellar activity index may be affected by mass accretion. The position of these
stars in the colour-magnitude diagram and the correlation with the amplitude of
the photometric rotational modulation is also scrutinised. Three regimes of the
chromospheric stellar activity are identified, confirming suggestions made by
previous authors on much smaller $R'_{\rm HK}$ datasets. The highest stellar
activity regime is associated with PMS stars and RS CVn systems, in which
activity is enhanced by tidal interaction. Some evidence of a bimodal
distribution in MS stars with $T_{\rm eff}\ge$ 5000 K is also found, which
defines the two other regimes, without a clear gap in between. Stars with 3500
K$\le T_{\rm eff} \le$ 5000 K are found to be either very active PMS stars or
active MS stars with a unimodal distribution in chromospheric activity. A
dramatic change in the activity distribution is found for $T_{\rm eff}\le$3500
K, with a dominance of low activity stars close to the transition between
partially- and fully-convective stars and a rise in activity down into the
fully-convective regime.
