They most certainly will not be taking one out of the wild. There are captive breeding programs for most primates and all of the great apes. The one that was shot was born at the Toronto zoo.
As far as i know Gorilla re-introduction programs do not work, gorillas are highly social animals and live in close knit family troups, it is extremely difficult to introduce an adult gorilla to a troup even in captivity where conditions can be controlled, in the wild a new gorilla would most likely be severly wounded or even killed by the first troup they ran into. Even introductions of baby gorilla's in capivity are hit or miss. The toronto zoo tried unsuccessfully to introduce a 1 year old back around 2000 to their troup.