From the BBC: Report on Gaia Mission Data 14th September 2016
Astronomers working on the Gaia space telescope have released a first tranche of data recording the position and brightness of over a billion stars. And for some two million of these objects, their distance and sideways motion across the heavens has also been accurately plotted. Gaia's mapping effort is already unprecedented in scale, but it still has several years to run.
Up to now, and until Gaia's results are fully analysed I guess, we have had to assume that the Milky Way was like other similar mass and group galaxies such as the one illustrated above.
From Martin's answer below Esa Report on Gaia mission
GAIA repeatedly measuring the positions and multi-colour brightnesses of all objects down to V = 20th magnitude (400 000 times fainter than the human eye can see). On-board object detection will ensure that variable stars, supernovae, transient sources, micro-lensed events and minor planets (including those that cross the orbit of the Earth) will all be detected and catalogued to this faint limit. Final accuracies of 10 microarcsec at 15 mag, comparable to the diameter of a human hair at a distance of 1000 km, will provide distances accurate to 10 percent as far as the Galactic Centre, 30 000 light-years away. Stellar motions will be measured even in the Andromeda galaxy more than 2.5 million light-years away, and tens of thousands of extra-solar planets will be discovered.
My specific questions, to limit an otherwise broad list are, will we know more about:
The influence of a central black hole?
Possible dark matter effects, but I don't know if looking for a halo was included in it's mission?
A much accurate idea of the region around the solar system?
Any other points that are deemed important would be appreciated. I do realise that it will take years for a full analysis of all the data, and Gaia's results may raise more questions than answers.