Originally Posted by Nereid

If you click on the link in the OP, then the ESA PR, you will find a wealth of material about this mission.

This PDF contains a summary of the expected accuracy for parallax, position, and proper motion. At its best (bright red stars), Gaia will measure distance (from parallax) out to ~100,000 parsecs; for stars ~10,000 pc away, with a Vmag of ~<15, Gaia will measure their distances accurate to ~10-20%.

The effects of 'gravity on light' will be calculated, using accurate data on the spacecraft's position, the mass of the Earth, etc.

Gaia aims to measure the proper motion of the stars it observes. It will make ~90 observations of each of the ~1 billion stars, over its 5 year lifetime.