  We measure the total stellar halo luminosity using red giant branch (RGB)
stars selected from Gaia data release 2. Using slices in magnitude, colour and
location on the sky, we decompose RGB stars belonging to the disc and halo by
fitting 2-dimensional Gaussians to the Galactic proper motion distributions.
The number counts of RGB stars are converted to total stellar halo luminosity
using a suite of isochrones weighted by age and metallicity, and by applying a
volume correction based on the stellar halo density profile. Our method is
tested and calibrated using Galaxia and N-body models. We find a total
luminosity (out to 100 kpc) of L_halo = 7.9 +/- 2.0 x 10^8 L_Sun excluding Sgr,
and L_halo = 9.4 +/- 2.4 x 10^8 L_Sun including Sgr. These values are
appropriate for our adopted stellar halo density profile and metallicity
distribution, but additional systematics related to these assumptions are
quantified and discussed. Assuming a stellar mass-to-light ratio appropriate
for a Kroupa initial mass function (M*/L = 1.5), we estimate a stellar halo
mass of M*_halo = 1.4 +/- 0.4 x 10^9 M_Sun. This mass is larger than previous
estimates in the literature, but is in good agreement with the emerging picture
that the (inner) stellar halo is dominated by one massive dwarf progenitor.
Finally, we argue that the combination of a ~10^9 M_Sun mass and an average
metallicity of <[Fe/H]> ~ -1.5 for the Galactic halo points to an ancient (~10
Gyr) merger event.
