[1] There is more than one way to model GR or large subsets of it either mathematically or physically. It sounds like a complex theory but in many ways it is actually "minimal" and highly constrained, making it pop out of unrelated things surprisingly easily. For example, crystal defects moving under thermal vibrations of the lattice can model GR! Similarly, variable index of refraction has very GR-like mathematics and can model everything except torsion (I believe, I'm not an expert).
[2] A key thing with such fundamental theories is not just to show that it can do something in physics but that it cannot do anything else.[2b] Without that constraint, any general computational theory like Turing machines "contains physics". So does the space of all computer programs, lambda calculus, etc... Those aren't useful statements, but that's pretty much what Wolfram's theories boil down to. He keeps finding very simple systems that can compute arbitrary things, pointing at them and exclaiming that "Physics is in there... somewhere!"
[2b] E.g.: Four dimensions of spacetime with a +++- or equivalently a ---+ signature, but not anything else such as ++++ or ++-- or five dimensions. Similarly, three generations of particles, not two or more than three. Etc...
