  Context. The second Gaia data release (Gaia DR2) contains high precision
positions, parallaxes and proper motions for 1.3 billion sources. The resulting
Hertzsprung-Russel diagram reveals fine structures all over the mass range.
  Aims. This paper aims to investigate the content of Gaia DR2 at the low-mass
end and to characterize ultra-cool and brown dwarfs.
  Methods. We first retrieve the sample of spectroscopically confirmed
ultra-cool and brown dwarfs in Gaia DR2. We use their locus in the precise
Hertzsprung-Russel diagram to select new candidates, and to investigate their
properties.
  Results. The number of spectroscopically confirmed objects recovered in Gaia
DR2 corresponds to 62 %, and 71 %, of the expected number of objects with and
estimated Gaia magnitude Gest $\leq$ 21.5, and 20.3, respectively,
significantly filling the gap with Gaia DR1. Furthermore Gaia DR2 contains ~13
000 $\geq$M7 and 679 new L candidates. A tentative classification suggests that
a few hundreds are young or subdwarf candidates. Their distance distribution
shows that the solar neighborhood census is still incomplete.
  Conclusions. Gaia DR2 offers a great wealth of information on the low-mass
objects. It provide a homogeneous and precise catalog of candidates worthwhile
to be further characterized with spectroscopic observations.
