Marcel Grossmann is perhaps best known for helping Einstein learn the Riemannian geometry necessary to formulate general relativity. He was instrumental in its early development. Wikipedia states:
The collaboration of Einstein and Grossmann led to a ground-breaking paper: "Outline of a Generalized Theory of Relativity and of a Theory of Gravitation", which was published in 1913 and was one of the two fundamental papers which established Einstein's theory of gravity.
The Wikipedia page on Grossmann references that paper and a second paper the duo authored the presented the second part of general relativity, "Covariance Properties of the Field Equations of the Theory of Gravitation Based on the Generalized Theory of Relativity".
This provides an English translation of the first paper. Note that Einstein is credited with the "physical part", while Grossmann is credited with the "mathematical part". This suggests that Grossmann was not intimately tied to the theory's physical predictions, merely the math behind it.
Did Grossmann author any other papers on general relativity, with or without Einstein?