Gaia DR2 giants in the Galactic dust -- II. Application of the reddening
  maps and models
  We exploit a complete sample of 101\,810 {\it Gaia} DR2 giants, selected in
Paper I in the space cylinder with a radius of 700 pc around the Sun and a
height of $|Z|=1800$ pc, using the {\it Gaia} DR2 parallaxes, $G_\mathrm{BP}$
and $G_\mathrm{RP}$ photometry, and {\it WISE} $W3$ photometry. We explain the
spatial variations of the modes of the observables
$G_\mathrm{BP}-G_\mathrm{RP}$ and $G_\mathrm{RP}-W3$ by the spatial variations
of the corresponding reddenings described in the GM20 3D dust distribution
model. Presented in this paper, GM20 is an advanced version of the model
introduced by Gontcharov in 2009. GM20 proposes two intersecting dust layers,
along the Galactic mid-plane and in the Gould Belt, with exponential vertical
and sinusoidal longitudinal variations of the dust spatial density in each
layer. The Belt layer is an ellipse, oriented nearly between the centre and
anticentre of the Galaxy, and with a semi-major and semi-minor axes of 600 and
146~pc, respectively. $G_\mathrm{BP}-G_\mathrm{RP}$ and $G_\mathrm{RP}-W3$ give
similar solutions, but different equatorial layer scale heights of $150\pm15$
and $180\pm15$~pc, respectively, and
$(G_\mathrm{BP}-G_\mathrm{RP})_0=(1.14\pm0.01)-(0.022\pm0.010)\,|Z|$,
$(G_\mathrm{RP}-W3)_0=(1.44\pm0.01)-(0.015\pm0.010)\,|Z|$, where $Z$ is in kpc.
We compare GM20 with several 3D reddening models and maps in their ability to
predict the observed colour modes. GM20 and the 3D map by Gontcharov appear to
be the best among the models and maps, respectively. However, the most reliable
models and maps mainly disagree only in their estimates of low reddening,
including the reddening across the whole dust layer.
