We report from ab initio calculations a magic magnetic cage cluster of gold, Gd@ Au15, obtained by doping of a Gd atom in gold clusters. It has a highest occupied molecular orbital-lowest unoccupied molecular orbital gap of 1.31 eV within the generalized gradient approximation that makes it a potential candidate for cancer therapy with an additional attractive feature that its large magnetic moment of 7 $\mu$B could be beneficial for magnetic resonance imaging. {\textcopyright} 2010 American Institute of Physics.