Gadolinium is an element, number 64 on the periodic table.
Spock's memory test taken in 2285 asked him to specify the electron configuration of gadolinium. (Star Trek IV: The Voyage Home)
According to the script, his answer was 5s22s22p63d104s24p64d104f75s25p64d16s2.
For the majority of stars, the rare element europium was enriched relative to gadolinium and samarium. (TNG: "Manhunt")