It is definitely difficult to see the gusset in that particular sock. This site
has a close-up on a gusset, and the stitches below the gusset are purled, so it's really easy to make out the triangle. Here's
another sock where the gusset is more visible. (You can see the top two edges of the triangle easily.)
I'm just being nit-picky about the terminology. Practically speaking, that diagram is all you need to think about, because it shows the whole section that is knit while working the gusset. What defines the gusset is a decrease that is worked on both sides of the sock, in this section, which creates that diagonal line sloping down from the top corner of the heel flap. The gusset stitches are decreased, to eat up extra stitches created when you pick up stitches along the sides of the heel flap. Without the gusset decreases, you would have a huge foot of the sock! LOL.