Gaia DR4 marks the culmination of the mission’s nominal five-year survey. It will deliver full-mission epoch-level astrometry, photometry, and spectroscopy for nearly two billion sources, offering a fundamentally new view of the Milky Way's stellar populations, structure, and dynamics. DR4 will include a comprehensive catalog of non-single stars and exoplanet candidates, opening multiple-star systems, compact object companions, and orbiting planets to population-scale study at a level previously reserved for targeted surveys. The scale and richness of this release demand rapid, collaborative exploration if the community is to translate raw data into scientific insight in the critical weeks after release.

The 2026 Gaia Relay is both a scientific gathering and an experiment on how we organize conferences. It is one event, with three legs: participants, projects, and interim results will all be different in the different legs, but we hope that ideas and collaborations will extend from one leg to the next. Central questions include: What does epoch-level astrometry and spectroscopy reveal that static catalogs could not? How do we rapidly vet and characterize the new non-single-star and exoplanet candidate catalogs? What tools and visualizations best expose structure in a data set of this scale and dimensionality? And can a relay format, distributed across sites and sequential in time, become a model for how the field responds to major survey releases in the future?