Christoph- I’m not sure what you mean by “found out”. The Cross River gorilla was first described as a different species in 1904 by zoologist Paul Matschie, based on skull and tooth differences from a few specimens. Subsequent classifications found that they were not a new species, but rather a subspecies and they were classified as Gorilla gorilla Diehli. Harold Coolidge redifined the genus Gorilla in 1929 to make them all one species which ended up leading to the eventual classification of the Cross River gorillas as just local populations of Western lowland gorillas Gorilla gorilla gorilla.

Note that gorillas are currently seen as two species, Eastern gorillas, and Western gorillas. Anyway for a long time the Cross River gorillas were seen as the same as the same as Western lowland gorillas, which themselves would now be considered a subspecies of Western gorilla. Confused yet?

In 2001, Sarmiento and Oates took another look another look at the distinctive features of this gorilla and revived them as a separate subspecies of the Western gorilla, but it is Matschie who actually first “found out” they were different than other gorillas and described them as such.

It’s interesting also that after the war in Nigeria in the 1960s, the Cross River gorilla was considered extinct until it was rediscovered in the 1980s.