Great diagram, Beldaraan! Did you make it?! Cool!
A minor clarification... Technically, the "gusset" itself is actually just the little triangular wedge shape that sits next to the heel flap on each side, and tapers the heel flap down to nothing. It's in the section you've correctly highlighted, but doesn't constitute that whole section. But you do
typically knit that whole section, in the process of working the gusset, so your diagram makes perfect sense, practically-speaking!
Very helpful diagram! I love it! :D