Emacs's native scroll-bar (--without-toolkit-scroll-bars) can have a
huge gap between fringe and scroll-bar. To see this:

  emacs -Q -l sb.el

Where sb.el has:
,----[ sb.el ]
| (setq initial-frame-alist
|       '((scroll-bar-width . 10)))
`----

As you can see the gap between the fringe and scroll-bar is close to
8 pixels. I see no gap for gtk scroll-bar though.

Screenshot: