"Tall Tales" was the first encounter Dean and Sam had with Gabriel, but not the first he had with them. When he heard about them, Gabriel actually followed the Winchesters invisibly for a few months. He'd planned to kill them off with a well-placed bear or vampire, to stop the Apocalypse, stop his brothers from fighting. Of course, being the Archangel of ADHD that he is, he got distracted by their adventures and their entertainment value. Especially when the "prank wars" started up- he silently cheered every time Sam one-upped Dean. He even started influencing a few things; nothing major at first, just the occasional "oh gosh, that werewolf that was about to gut Sam has a sudden case of Head Exploding For No Reason" or "Sam's motel room happens to have a jacuzzi after a long, hard day". And, well, if all his little nudges sort of revolve around Sam, chalk it up to the fact that someday in the near future Dean's gonna get his own Angel. So what if Gabriel starts hiding extra clips of ammo and the odd twenty dollar bill in Sam's bag- he's just balancing the cosmic scales.
He's completely unaware (or at least completely in denial) of how hard he's falling until it's much too late.
Then, one day, when he's just come back from placing a mattress under the window that Sam will fall out of tomorrow during a hunt, he is shocked to sense one of his brothers. Not Castiel- some obscure Angel, not even after the Winchesters, but curiously investigating the traces of Gabriel's grace that he's been using unconsciously. The Trickster barely evades detection. The smart thing- the normal thing- to do in this situation would be to run off to Cambodia or Iceland for a while, and to never, ever engage the Winchesters again.
Right. Like that's gonna happen.
The next evening, once the two brothers have bedded down (after Sam fell out of the window and onto a surprisingly clean, springy mattress), Gabriel sticks his head into their room with the intention of saying goodbye and maybe leaving a few parting gifts. He's just finished short-sheeting Dean's bed and filling both brothers' shoes with M&Ms and is sneaking back toward the door when he hears a gasp. He turns quickly, expecting to find himself face-to-face with a furious Winchester and at least one instrument of death pointed at him. Instead he sees the prone, curled form of Sam, whimpering and clutching at his pillow as he battles some nightmare or other. Without thinking, Gabriel is crouched next to his bed, two fingers to the sleeping behemoth's forehead, and Sam's expression clears with a soft sigh.
Gabriel catches himself smiling gently.
That's when he realizes he's in trouble.