  The Gaia astrometric mission may offer an unprecedented opportunity to
discover new tidal streams in the Galactic halo. To test this, we apply nGC3, a
great-circle-cell count method that combines position and proper motion data to
identify streams, to ten mock Gaia catalogues of K giants and RR Lyrae stars
constructed from cosmological simulations of Milky Way analogues. We analyse
two sets of simulations, one using a combination of $N$-body and
semi-analytical methods which has extremely high resolution, the other using
hydro-dynamical methods, which captures the dynamics of baryons, including the
formation of an in situ halo. These ten realisations of plausible Galactic
merger histories allow us to assess the potential for the recovery of tidal
streams in different Milky Way formation scenarios. We include the
Gaia~selection function and observational errors in these mock catalogues. We
find that the nGC3 method has a well-defined detection boundary in the space of
stream width and projected overdensity, that can be predicted based on direct
observables alone. We predict that about 4-13 dwarf galaxy streams can be
detected in a typical Milky Way-mass halo with Gaia+nGC3, with an estimated
efficiency of $>$80\% inside the detection boundary. The progenitors of these
streams are in the mass range of the classical dwarf galaxies and may have been
accreted as early as redshift $\sim3$. Finally, we analyse how different
possible extensions of the Gaia mission will improve the detection of tidal
streams.
