See, I've been having some thoughts on that. Because you're right, it shouldn't exist here, a lot of things shouldn't exist here but they do. So... yeah, been thinking.
In theory, it's entirely possible that by bringing people in from the reality where the Force is a real thing, that Gems have somehow dragged the Force in as well, manipulating time and reality so that it's always existed here. I mean, eventually you have to ask when this all started because you're getting into the Grandfather Paradox but...
Basically, it wasn't here. Then people brought it here with them. The Gems brought it here with the people who needed it.
Well, that or you're looking at a multi-reality bleedthrough of different powers but that could lead to the idea of all of them collasping in on each other because there's too many to be sustained, but that's a bit too doom and gloom for my liking.