We report the first electron affinity measurements for some endohedral gadolinium metallofullerenes. Knudsen cell mass spectrometry has been used for the study of the gas-phase composition over the KF-Gd@C-n system. Both neutral and negatively charged species C-60, C70+2n (n = 0, 1, ..., 10), Gd@C-74, Gd@C-76, Gd@C-78, Gd@C-80, and Gd@C-82 were detected at 931-960 K, which permitted determination of equilibrium constants of electron exchange reactions. The electron affinities of gadolinium metallofullerenes were found to be greater than those of the parent fullerenes. In contrast to the empty fullerenes, endohedral metallofullerenes revealed no strong dependence of their electron-accepting ability on the size of the fullerene cage.