  We use ESA/Gaia astrometry together with SEGUE and LAMOST measurements of the
GD-1 stellar stream to explore the improvement on the Galactic gravitational
potential that these new data provide. Assuming a realistic universal model for
the dark matter halo together with reasonable models of the baryonic
components, we find that the orbital solutions for GD-1 require the circular
velocity at the Solar radius to be $V_{\rm circ}(R_\odot) =244 \pm 4\,{\rm km\,
s^{-1}}$, and also that the density flattening of the dark halo is
$q_{\rho}=0.82^{+0.25}_{-0.13}$. The corresponding Galactic mass within 20kpc
was estimated to be $M_{\rm MW}(<20kpc)=2.5 \pm 0.2 \times 10^{11}\, M_\odot$.
Moreover, Gaia's excellent proper motions also allowed us to constrain the
velocity dispersion of the GD-1 stream in the direction tangential to the line
of sight to be $<2.30\,{\rm km\, s^{-1}}$ (95 % confidence limit), confirming
the extremely cold dynamical nature of this system.
