One of my friends went to Borneo and saw the attempts to release orang-utans into the wild. One of the main problems is that the released orangs kept associating people with food and would return in the evening. Generally speaking, there have been relatively few successes in releasing captive animals into the wild. Luckily, one group of golden lion tamarins were tested for an illness, just before being released. If they hadn't been tested, their diseases could have severely affected wild tamarins.
I don't know how much effort Damian Aspinall put into his research, but he couldn't have foreseen what was going to happen to the released gorillas. If there is no intention to release captive gorillas into the wild, why are over 700 kept in zoos? The reason tends to be financial, rather than conservation.
I agree with Yassa that released captive animals need enough space, food etc and must not compromise the wild population. I also agree that it is better to translocate wild animals, rather than spending money trying to train captive animals to live in the wild.
I also agree that western lowland gorillas probably do benefit from the release of captive animals into existing populations. As there may be 100.000 gorillas left in the wild, it would be better to spend money on protecting the gorillas and their natural habitat, rather than building another expensive enclosure to attract visitors, while also leading to other animals no longer being kept in the collection. That is one reason why I joined the RSPB, as it had paid for an area of rainforest in Sumatra, which will benefit Sumatran rhinos, tigers, orang-utans and other species far more than an expensive new enclosure in a zoo.