It's only available on Steam so far, here is the announcement.
Bring it to GOG, vote here for King's Quest.

I keep forgetting this game exists.
I should pick it up when all of it is out.

It was purchasable before right? So people who bought it before are fine with it going free now?

A friend told me he got it for 0.99. It had a -90% sale for the first episode.
Edit: Different avatars? What?

That's like asking if people who bought a game for $10 are fine that it goes on sale for $5. Yes, we feel a slight sense of loss, but that's just the way things go. The more you wait, the lower prices get, most of the time.

I was tempted to get chapter 1 during that 90% off sale, luckily I didn't.
Ah GOG identified my secret affiliation with organization fighting aliens. :P Seems like a recent problem where avatar doesn't update properly in forums. Ah I suppose you are right, I felt that being free is not the same thing as going on a sale since it is generally expected that a game will be on sale if someone waits but it is not normal for a game to go on to become free if someone waits on getting it.

From what I've seen & read about it, I don't even want this one for free.

I do, it seems great.
Really weird that the publisher didn't even mention the game on Twitter or anything when the latest episode released though.

I think this is how episodic gaming should've always been done, like shareware: a full episode for free.

They've done this before, pretty sure the IOS versions of the Telltale games offer the first episode for free.
(might have been limited offers)

Even non-iOS. I remember getting the first episodes of Sam & Max free, and Tales of Monkey Island as well.

It's more common than you think. Even here on GOG games are given for free and then continue to be sold for full price (for example Fallout was my first purchase here and was later given for free). Many other commercial games are available for free on various sites. It's less often that a game goes completely free, but also happens (and IMO better than a temporary free game, where those who miss the deal can be very disappointed).

Is it an episodic game?

Yes.

Telltale also did this with Sam & Max Season1 on PC, oddly though they choose Episode 4 as the free episode.