Half Life fans have been waiting for Half Life 3’s announcement for more than a decade now and they are expecting the wait to end this year. For the majority of the fans, they believe that Valve is going to announce on Half Life 3 at E3 but for the minority, they are hoping for GDC.
We hate to be the mood-killer in this drama but there is simply no way for Half Life 3 to get announced at GDC. This is because GDC is more of a tech exhibition, not games. Valve will most probably make full use of their time at GDC to discuss more on their VR headset project instead.
Besides, Half Life 3 is going to be a global phenomenon when released and GDC is too small of an event to host the announcement of the game. If there is an ideal place for Half Life 3 to get announce, it is either on E3 or a surprise event organized by Valve.