There are 7 naturally occuring isotopes of Gadolinium, Gd-152, 154, 154, 155, 156, 157, and 158 with Gd-158 being the most abundant. Gd-152 is the only naturally occuring isotope that is radioactive, but it only makes up about 0.2% of natural Gd. It alpha decays with a half-life of 1.08E+14 years ...