How precisely we know the Gaia spacecraft baseline? [Question related to Planet 9 and the cosmic distance ladder]
This page of the documentation sort of answers the question.

An important part of Gaia data processing is the Gaia ephemeris allowing one to compute Gaia position and velocity in the BCRS for any moment of time covered by observations. Clearly, the accuracy of Gaia ephemeris is crucial for the project. The required accuracy of Gaia velocity is driven by the aberration of light: 1 μas in direction corresponds to about 1 mm s−1 in velocity. The requirements for the accuracy of Gaia position comes from the paralactic effect for the near-Earth objects (NEOs) and was assumed to be 150 m. The latter requirement is only important for data releases that include solar-system data.

So the position is known to 150m and the velocity to a few millimeters per second. This is probably relative to the Earth, but the Earth's orbit is known to comparable precision. In fact a far less precise determination of the position would do for observing the parallax of stars. The largest such parallax is about 1 second of arc and the Gaia's observations of the position of stars is accurate to a few micro arc seconds, so the errors there are at least a few parts per million. On the other hand an error of (say) 1km in the diameter of the orbit is a few parts per billion. The precision location data is only needed for observing objects within the solar system.