Kumbuka, a western lowland silverback, made an "opportunistic" exit through unlocked doors, London Zoo added.
The zoo said the gorilla got into a secure keepers' area on discovery of the security flaw.
Kumbuka was returned to his den shortly after being tranquilised on 13 October.
What happens if you drink five litres of undiluted squash?
The 29st "alpha male" was kept calm by a keeper before the keeper managed to remove himself from the area.
Armed police were called to the zoo and visitors were evacuated after the ape's bid for freedom.
You might also like:
Handsome Pakistani 'chai wala' turns model after trending on Twitter
Hot dogs 'must be renamed' in Malaysia, says religious body
Celtic fan, 5, phones club to apologise for missing match
But in his report on the breakout, Prof David Field described the events as "less dramatic than some would have you believe".
He said: "There were no broken locks, Kumbuka did not smash any windows, he was never 'on the loose' and his normal gorilla posturing reported by visitors earlier in the day was unrelated."
The gorilla found the door to his area was unlocked and a second door had yet to be secured shortly after his feeding time at 17:00 BST last Thursday.
It brought the gorilla face to face with the keeper who had shared an "incredibly close relationship" with the animal.
The member of staff talked to Kumbuka and "removed himself from the area".
Once alone, the animal "briefly explored the area next door to his den, where he opened and drank five litres of undiluted blackcurrant squash", the report revealed.
It concluded that the gorilla was always contained in a non-public area of the zoo after the alarm was raised, in accordance with its policy.
Kumbuka was returned to his den shortly after being tranquilised.
A gorilla that escaped from its enclosure at London Zoo drank five litres of undiluted blackcurrant squash before being returned to his den, a report has found.