The satellite was launched in December 2013 followed by a six-month long in-orbit commissioning period. It began routine scientific operations on 25 July 2014 and for 28 days, operated in a special scanning mode that sampled great circles on the sky, but always including the ecliptic poles.
This meant that the satellite observed the stars in those regions many times, providing a database for Gaia’s initial calibration for its main survey operation which began on 21 August 2014.
The European Space Agency writes:
The Gaia team have spent a busy year processing and analysing these data, en route towards the development of Gaia’s main scientific products, consisting of enormous public catalogues of the positions, distances, motions and other properties of more than a billion stars. Because of the immense volumes of data and their complex nature, this requires a huge effort from expert scientists and software developers distributed across Europe, combined in Gaia’s Data Processing and Analysis Consortium (DPAC).
“The past twelve months have been very intense, but we are getting to grips with the data, and are looking forward to the next four years of nominal operations,” says Timo Prusti, Gaia project scientist at ESA.
“We are just a year away from Gaia’s first scheduled data release, an intermediate catalogue planned for the summer of 2016. With the first year of data in our hands, we are now halfway to this milestone, and we’re able to present a few preliminary snapshots to show that the spacecraft is working well and that the data processing is on the right track.”
There is more about the mission on the ESA website.
