Any data that is provided to support the use of Gaia data or explain the Gaia Data Processing and Analysis in more depth are summarised below:
The photometric system of Gaia
Auxiliary data on Astrometry: Considerations for the use of DR2 astrometry
Find auxiliary data provided by L. Lindegren along with his presentation "Considerations for the use of DR2 astrometry" here.
Auxiliary data on crossmatches
Find auxiliary data on Gaia DR2 crossmatch: Hipparcos2 here.