I actually think the end was kinda rushed, the way Serena forgives Dan was really pushed. About the Gossip Girl identity, the last season really blew it up, we can easily guess who is behind all that.
While it was related to school, Gossip Girl was one of the best series I've seen, but after that, it's just another one.
Easily predictable, almost no emotional scenes and some things don't fit together I suppose because the story line wasn't good enough.
But I liked it anyway.