>>Is it true or not that Third class was locked behind gates? I've seen a powerpoint that tries to debunk this theory, but I haven't seen any more on it. <<
it did more then debunk the locked gates story, it nuked it. I know the lady who researched it and her sources. The locked gates you hear about in the inquiries are the waist high gates at the top of the stairs leading up to B Deck from the Well Deck. One of the third class passengers easily vaulted right over it.
The problem with the locked gates is that the Bostwick type gates portrayed in the film were known to exist only in three places. One was in a ladder up in the forward 3rd Class acommodation and since this was flooded early on, it's of no relevance. Nor is the one which, oddly enough, was in the vegetable prep area. The third one, and this has been photographed on the wreck, barricaded the shell doors on the hull in the 1st class area so again, is of no relevance.
While this doesn't prove 100% that there weren't others, they aren't documented and they were no barrier to anybody trying to get outside. All anyone had to do was go up.