Grossmann's contribution to the Entwurf theory consisted in the following.This is a substantial part of the theory, but Einstein wrote privately:
* He clarified the mathematical foundation of the theory based on a general line element (8) and generalized the concept of a tensor to a structure of n th rank in m-dimensional space.
* He identified the absolute differential calculus by Ricci and Levi-Civita as the relevant mathematical toolbox for the problem of a relativistic theory of gravitation and transformed it into a tensor calculus both with respect to notation and by generalizing it to mixed tensor densities.
* He proved that the conservation law for matter (11) was a generally covariant equation by showing that it expresses the covariant divergence of Theta-mu-nu.
* He identified the Riemann tensor as a relevant and rich resource for the problem of constructing generally covariant gravitational field equations, and he showed Einstein several ways of extracting a second rank object from the Riemann tensor that would have the desired limiting form in the case of weak static fields.
* After the failure of the mathematical strategy of constructing a field equation from the Riemann tensor, he proved the central identity (39) from which the gravitational field equations of the Entwurf theory were read off.
* In joint work with Einstein, he showed how the Einstein-Grossmann theory can be formulated in terms of a variational principle and clarified its transformational properties in light of the hole argument
Grossmann will never lay claim to being co-discoverer. He only helped in guiding me through the mathematical literature but contributed nothing of substance to the results.Grossmann ought to be considered a co-discoverer. After the collaboration, Einstein wrote nonsense papers for over a years, and did not get back on track until he collaborated with Hilbert in 1915.
It appears that Grossmann and Hilbert are not credited because of a prejudice against mathematicians. Their contributions are considered to be just math, not physics. But finding the equations of general relativity was largely a mathematical problem, given Newtonian gravity and special relativity.