The launch of Gaia is officially delayed due to a potential technical issue with the satellite that has to be resolved first. No new launch date is given yet as the new integrated schedule has to be fixed first. Here is the announcement by ESA: http://www.esa.int/Our_Activities/Space_Science/Gaia/Gaia_launch_postponed .
The announement of Arianespace was published: http://www.arianespace.com/news-press-release/2013/10-22-2013-Gaia-launch-postponed.asp
The launch will be delayed at least until December 17, when another launch window for Gaia will open as reported by Isabelle from Astrium, Toulouse: http://reves-d-espace.com/2013/10/22/report-du-tir-gaia/ .
And we have a statement in the ESA Gaia blog: http://blogs.esa.int/gaia/2013/10/22/gaia-launch-postponed/
There is not much information given in all these statements. It seems to be that there is a serious technical issue with the Gaia satellite that cannot be fixed within a few days. The launch window that was aimed for is ending on December 5. The next window available starts on December 17, the NET date given in one of the documents linked above. Looking at the launch calender of Arianespace shows a launch of an Ariane 5 with two communication satellites planned for mid-December. If this launch of the Ariane 5 will really happen in mid-December (depends mainly on if both satellites will be ready) then Gaia cannot be launched until the end of December. This assumes that a much more profitable Ariane 5 launch is preferred by Arianespace than a launch of a smaller Soyuz-Fregat. The process of switching from one launcher to another one at Kourou is known to take two weeks or more. The range system (that controls the path of the rocket after launch and allows to take actions if the planned trajectory is not followed) has to be heavily modified and retested.
As you can see the entire process of deciding when a new launch of Gaia can happen is really complicated as it depends on many facts like the given ones above. Well, there could be more things not publically known that complicate things even more. Let us hope there will be a launch in late 2013 or early 2014. It may take some time until all actions and decisions necessary are finalized and a new launch date can be published.
Update: Some details were published today about the issue leading to the delay of the Gaia launch at http://www.spacenews.com/article/launch-report/37826component-alert-forces-esa-to-postpone-gaia-launch. Thales Alenia Space Italy has issued a component alert after problems were discovered with the same component built into another satellite (believed to be a satellite of the O3B constellation according to post on this forum http://forum.nasaspaceflight.com/index.php?topic=19838.90). The component in question is built into the transponders of Gaia. These components will be removed and shipped back to the manufacturer in Italy for repair or replacement. The entire process will take at least four weeks including reinstallation and testing. If the process does not take longer then a launch on December 17 or later in the launch window could be possible – if Arianespace is agreeing. The latter depends on the actual launch date of the Ariane 5 in December.
Update 2: Arianespace has just published that the Ariane 5 ECA for the launch in December was transferred to the Final Assembly Buiding (FAB) of the European spaceport. This is a rather early movement for a launch planned in mid-December. Does this have any importance for the Soyuz-Fregat launch of Gaia? We will see.
Here is the link for the Arianespace press release: http://www.arianespace.com/news-mission-update/2013/1103.asp
Stay tuned for additional updates.