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 circ (R) = 244 ± 4 km s −1 , and also that the density flattening of the dark halo is q ρ = 0.82 +0.25 −0.13. The corresponding Galactic mass within 20 kpc was estimated to be M MW (< 20 kpc) = 2.5 ± 0.2 × 10 11 M. 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 km s −1 (95% confidence limit), confirming the extremely cold dynamical nature of this system.