Update: The AMA is going on now. Find it here.
I imagine Gabe Newell spent much of last week with his ears burning following Game Informer's interview with an alleged anonymous Valve employee. The source, whose opinions interviewer Andrew Reiner was unable to verify, suggested Half-Life 3 did not exist, has never existed and probably won't ever exist—the same applying to Half-Life 2: Episode 3. Tomorrow the world has the chance to hear it straight from the horse's mouth as Gabe Newell has agreed to host a Reddit AMA.
I imagine the veracity of the Game Informer interview will be called into question but whether Newell goes into detail—or even acknowledges the question—remains to be seen. Questions relating to Half-Life will almost certainly dominate the conversation, however with Dota 2's comprehensive 7.0 update having recently landed, among other Valve-related things, there will be plenty to talk about over and beyond Gordon Freeman et al.
In any event, I guess we'll find out tomorrow. Check back afterwards for the skinny on what Newell has to say.