At first I had thought this was just a store announcement, but it appears that it is showing up on other GW store sites as well. The only bits of information that I am currently able to find is that....
It is not a chaos codex, and that there are no pictures because none were shown for whatever GW100 is.
Info that is coming in.....
May 2016, more information arriving soon....
100 days from today is May 14, which is a Saturday, i.e. a product release
Games Workshop: Freeway V
Games Workshop: Larwin Square