Nothing that I've ever seen of GR would contain an answer to where inertia comes from, unless I missed it. GR is a relativistic description of gravity, inertia is something different. You have to simply tack on inertia when doing calculations in GR, just as you had to with Newton's equations. All GR does is give you a way to understand why inertia and gravitational mass are the same, via the equivalence principle. No physical theory can explain itself, theories are assumptions that are used to explain other things.