Naturally occurring gadolinium (₆₄Gd) is composed of 6 stable isotopes, ¹⁵⁴Gd, ¹⁵⁵Gd, ¹⁵⁶Gd, ¹⁵⁷Gd, ¹⁵⁸Gd and ¹⁶⁰Gd, and 1 radioisotope, ¹⁵²Gd, with ¹⁵⁸Gd being the most abundant (24.84% natural abundance). The predicted double beta decay of ¹⁶⁰Gd has never been observed; only a lower limit on its half-life of more than 1.3×10²¹ years has been set experimentally.

Thirty-three radioisotopes have been characterized, with the most stable being alpha-decaying ¹⁵²Gd (naturally occurring) with a half-life of 1.08×10¹⁴ years, and ¹⁵⁰Gd with a half-life of 1.79×10⁶ years. All of the remaining radioactive isotopes have half-lives less than 74.7 years. The majority of these have half-lives less than 24.6 seconds. Gadolinium isotopes have 10 metastable isomers, with the most stable being ^143mGd (t_1/2 = 110 seconds), ^145mGd (t_1/2 = 85 seconds) and ^141mGd (t_1/2 = 24.5 seconds).

The primary decay mode at atomic weights lower than the most abundant stable isotope, ¹⁵⁸Gd, is electron capture, and the primary mode at higher atomic weights is beta decay. The primary decay products for isotopes lighter than ¹⁵⁸Gd are isotopes of europium and the primary products of heavier isotopes are isotopes of terbium.

Gadolinium-153 has a half-life of 240.4 ± 10 days and emits gamma radiation with strong peaks at 41 keV and 102 keV. It is used as a gamma ray source for X-ray absorptiometry and fluorescence, for bone density gauges for osteoporosis screening, and for radiometric profiling in the Lixiscope portable x-ray imaging system, also known as the Lixi Profiler. In nuclear medicine, it serves to calibrate the equipment needed like single-photon emission computed tomography systems (SPECT) to make x-rays. It ensures that the machines work correctly to produce images of radioisotope distribution inside the patient. This isotope is produced in a nuclear reactor from europium or enriched gadolinium. It can also detect the loss of calcium in the hip and back bones, allowing the ability to diagnose osteoporosis.

Gadolinium-148 would be ideal for radioisotope thermoelectric generators due to its 74-year half-life, high density, and dominant alpha decay mode. However, gadolinium-148 cannot be economically synthesized in sufficient quantities to power a RTG.