I actually think its a good idea, but success will depend on how sound the planning is. i.e. how much protection they will receive initially in an unstable area. Plus there's always the danger it might not work well with these partly humanised animals-unless they are given very stringent protection from hunting etc.
But either way I think the risk is worth taking and it will prove if it can be done with adult Gorillas which have lived a long time in captivity, or not. Howletts/PL between them now have five large breeding groups and continued successful breeding means they are nowadays at saturation point with Gorillas, as its fairly evident they don't propose to build yet more expensive accomodation for more groups. Coupled with that is the continual problem of what do with all the surplus males these groups produce. A number of males at Port Lympne(and others that have gone elsewhere) are already consigned to a permanent life living in all-male groups which I don't think is very natural or satisfactory for them. Djala himself has never liked the visiting public so attempting to rewild both him and his group might be a fairer option for him.
Without them trying to do this, I can't see the point of Howletts/PL continuing to keep breeding from all those Gorillas and it was always John Aspinall's aim from the start to 'breed then reintroduce' them so this outcome is the perpetuation/culmination of that ideal really.
Whether it will be successful, either entirely or partially, remains to be seen, and if so whether some other groups from Howletts might follow suit in due course. I am also sure the vacated 'Palace of the Apes' where this group lives will soon be filled again- either by a new breeding group, or more surplus males.