Riemann was a mathematician, not a physicist, and he was simply trying to create a generalized n-dimensional geometry for any curvature, not just the two-dimensional geometry of planes created by Euclid. Riemann was not trying to develop a theory of gravity. His work preceded that of Einstein by nearly 50 years years. For his part, Einstein was unaware of Riemann's work and learned of it from a mathematician friend to whom he appealed for help. It is a good example of how math developed for its own sake often finds a use in physical theory.
