The structures of two bis-ethylpyrrolidinoadducts of Gd3N@I-h-C-80, obtained by regioselective 1,3-dipolar cycloadditions, were elucidated by single crystal X-ray, visible-near infrared (vis-NIR) spectra, studies on their thermal isomerization, and theoretical calculations. The structure of the minor-bis-adduct reveals a C-2-symmetric carbon cage with [6,6] [6,6]-addition sites and with an endohedral Gd3N cluster that is completely flattened. This is the first example of a crystal structure of Gd3N@I-h-C-80 derivatives. The structure of the major-bisadduct was inferred by the vis-NIR spectrum being corresponded to the structure of a previously reported major-bis-adduct of Y3N@I-h-C-80 known to have an asymmetric [6,6][6,6]-structure. Based on experimental results showing that the minor-bis-adduct of Gd3N@IhC80 isomerized to the major-adduct, a possible second addition site was elucidated with support from density functional theory calculations.