Already bought the Anniversary edition from the Another World website a couple of years ago, but I'm curious about the 'making of' video that's included in the Gog version. May end up buying it again to support Gog. How long is the 'making of'?

I haven't bought AW here yet, but I assume that this is the same version that was included in my boxed version. That is 17 minutes and 53 seconds.
Perhaps someone who bought it here can confirm this.

Paradoks is correct, the video is just under 18 minutes long. Definitely worth purchasing from GOG; if not only to support this great site, but to also learn more about an incredibly unique and emotionally moving game. I remember playing this on the SNES (as "Out Of This World") when I was very young, never got to beat it until now. Quite an experience.

The Making Of video is an awesome look into the creation of the game, I recommended everyone to watch it.