Official GamesCon website: http://www.gamescom-cologne.com/en/gamescom/presse/index.php
Link on the website showing every game to be announced at the show:
Scroll down the .pdf, and look for Valve. One entry by Valve.
source article (where I found out about this, and apparently the .pdf was just released recently today): http://www.t3.com/news/half-life-3-confirmed-at-gamescon