I already played the first few expansions but still have this open quest from the main game.
As far as I remember:
I rushed through and came back to the gatekeeper definitely within time. And I had two dialogue options. The one for the elixir of youth and the other - which I chose - ended the main quest. I chose the good ending, so the gates are closed and I saw the ending text scrolling.
So I ended up with the elixir of youth, the quest is still open, but I can't get back to the gatekeeper.
Would even be fine with me if I could at least get it to the failed quests. But in the active-tab it's a little pain...