Thats bad news: http://blogs.esa.int/gaia/2013/10/22/gaia-launch-postponed/
I heard through the grapevine that the new launch window opens in mid December.
Now ESA officially tells about the reasons:
Yesterday, the decision was taken to postpone the launch of ESA’s Gaia mission after a technical issue was identified in another satellite already in orbit. Gaia shares some of the components involved in this technical issue and prompt notification of this problem has allowed engineers working on the final preparations for Gaia’s launch to take additional precautionary measures. The issue concerns components used in two transponders on Gaia that generate ‘timing signals’ for downlinking the science telemetry. To avoid potential problems, they will be replaced. The transponders will be removed from Gaia at Kourou and returned to Europe, where the potentially faulty components will be replaced and verified. After the replacements have been made, the transponders will be refitted to Gaia and a final verification test made. As a consequence of these precautionary measures, it will not be possible to launch Gaia within the window that includes the previously targeted launch date of 20 November. The next available launch window is 17 December to 5 January 2014. More details will be given as soon as they are available. The new launch date will be announced when the timeline for completing the additional work has been confirmed and the overall launch manifest of Arianespace has been established.