And they aren’t re-introducing them in general.

Gorillas (and other primates) are being reintroduced back into the wild. The point of gorillas being kept in captivity is to maintain genetic diversity for breeding programs to avoid inbreeding. If there isn’t enough genetic diversity and the wild population disappears, that species will go extinct.

Nature isn’t one giant Disney movie. There is a reason gorillas live longer in captivity than in the wild.