Magnetic anisotropy and magnetization dynamics of rare earth Gd atoms and dimers on Pt(111) and Cu(111) were investigated with inelastic tunneling spectroscopy. The spin excitation spectra reveal that giant magnetic anisotropies and lifetimes of the excited states of Gd are nearly independent of the supporting surfaces and the cluster size. In combination with theoretical calculations, we argue that the observed features are caused by strongly localized character of 4f electrons in Gd atoms and clusters.