How come "Gscatter" doesn't appear in the add-on list?

On a YouTube tutorial, I saw the person was using GScatter. So, I downloaded it from its homepage. When I tried “install from disk”, GScatter showed some sort of pop-up which I just clicked “yes” without reading, like a normal person. At first, I thought it was not installed, because it was not shown in the add-on list, so I repeated installing. I found that GScatter is on the right side bar, but it still was not listed in the add-on list.

Why is it so?

It sounds like you are talking about completely different addons. The Gscatter addon won’t show up under the Extension tab.