I was going along the lines of Nemesis being a deity, and Outsiders are more or less Old Ones and their minions, so a slight misstep in my wording.
What I'm saying is, I think the Outsiders were not always Outsiders, their removal from reality to beyond the gates is what made them Outsiders, and this removal is some process that may occur to different entities at different points in history.
Interesting idea. A bit our-universe-centric, though; it would seem to indicate that the Outside exists not in and of itself but purely in relation to our universe. I see no reason to assume that Outsiders don't come from the Outside.
(I think the use of the term "Outside" is part of the problem here. It does implicitly state that the Outside is defined by its relation to our universe. The problem, then, is one of lack of objective frame of reference; I suspect that "Outsiders" simply call their universe "the universe" and ours "the Outside", as the perspective of here-vs.-there [i.e., the inescapable omnipresence of subjectivity] defines the way we think about everything.)