by Elminster retrieved on Gorion's corpse states that "I urge thee to leave Candlekeep this very night, if possible".
Was it carried by a messenger, with the order of delivering it as soon as possible? Then I think that a more appropriate choice of words would have been "right after you read this letter", rather than "this very night", which is a bit too much deterministic and in real-time, considering travel time of envoys. It's not unreasonable to write "this very night" while being aware that the reader might receive the letter after a couple of days. But from Elminster I would expect something with a better sense of timing, since he's also worried about Gorion's fate as "the other side will move very soon".
Besides, there is no guarantee anyway that a messenger would have reached Candlekeep sound and safe, considering the hostile activity in the Sword Coast. It would have been much better if Elminster directly visited Gorion for an update about the current situation. He's a wizard, he can teleport anywhere and he's known to appear whenever and wherever he wants. In fact, he just appeared in front of Charname at the start of chapter 1 after he sensed that Gorion died but his ward was alive...
Then, was the letter delivered by Elminster in person, considering that we meet him nearby Candlekeep? e.g., he left the citadel by handshaking Gorion, telling him that he understands the situation but wishes to stay neutral, still giving him the advice to take care of himself, and giving a him a message with the hint of reading it in private. Or maybe I think that Elminster approached the gatekeeper, asking him to deliver a letter.
I think that this last one is the most likely scenario. It's not unheard that important people make an official visit and then depart leaving behind a message with further info or instructions for certain, selected friends. And it's not inconceivable too that someone in disguise carries something with the task of giving it to someone else.
I wonder if for someone like Elminster, and for a friend like Gorion, it would have been more reasonable to give the advice by mouth, rather than with a letter that could be read or stolen by others (Imoen in fact reads it). Maybe Elminster didn't feel the necessity to teleport directly inside Candlekeep, violating the restrictions and the magical seals.
Third option: he teleported the letter to Gorion. Magic AT&T.