Gd3N@C-80 endohedral fullerene, a starting material for a potential magnetic resonance imaging contrast agent, is investigated by high-frequency electron spin resonance (ESR) and SQUID magnetometry. The magnetic moments of the three Gd ions of the endohedral Gd3N molecule are ferromagnetically aligned at low temperatures and are uncorrelated at high temperatures. The 4 T broad 210 and 315 GHz ESR spectra measured at 2 K are well-described by a single transition between the lowest Zeeman levels of static molecules shifted by fine structure effects. At higher temperatures there is a gradual transition to a rotating state. At ambient temperatures the rotation frequency is much larger than the fine structure broadening, and a single ESR line is observed at a gyromagnetic ratio of g = 1.995.