Calculating the distance of objects using the Gaia DR1
The early 1990s Hipparcos mission
yielded parallaxes for 118000 stars (Hipparcos catalog) and positions without parallax for another 2.4 million (Tycho-2 catalog).
The Tycho-Gaia astrometric solution (TGAS) combines those data with preliminary Gaia observations to get 2 million parallaxes, "only" 17 times as many as Hipparcos, with better precision.
If this is a small fraction of the total number of star positions, it's not a deficiency; it's just easier to get a position than a parallax.
Lindegren et al. 2016 explain what they could or could not do with the first 14 months of Gaia observations.
Future Gaia data releases should have still more parallaxes.