The first time Gabriel hears it, it nearly knocks him of of his feet. There's deep heavy beat, a sound of a pair of wings being stretched and tested. But for a long flight, through worlds and worlds, dimensions or time itself.
That alone wouldn't be enough to cause him to almost lost his balance, fall down the Lady of Liberty to drown in the raging sea beneath him. He retreated here to think, to find refuge and answers, but he never expected to hear that certain sound again.
Gabriel turns his head and waits. Perhaps it had just been his imagination and he didn't hear his long lost brother archangel wings being completely unfolded.
The next beat proofs him wrong and while he has a better stand now, he still feels the earth tremble beneath him. His own wings quiver in anticipation, because of all things that could've happened, he didn't expect to Lucifer care about something enough to tell everybody - every angel – to fuck off.
And while he's miles and miles away he still can hear the protective snarl of his brother voice through a connection he deemed long dead.
He can still see the light of the morning star which begins to gather at the horizon, promising a new age for mankind.
He can still smell the slightly burned human flesh, which bears now a hand print somewhere, given by the morningstar and former devil to his new flight partner.
He can still sense the rustling of all the wings Lucifer possesses, ready to go home.
Maybe not yet or any time soon, but he will.
After he probably smote Zachariah, hugged Michael (they so will, Gabriel will see to that) and told the host they would need a new devil, but Lucifer would go back to heaven. And that's far earlier than the 'never' his elder brother threw at him, when he saw him last.
Gabriel knows this is in fact the most simplest solution to the apocalypse earth could've found, but he doubts humanity ever realize how close to widespread destruction their little planet came. Humanity will never know how unlikely it was to Lucifer actually claim Sam fucking Winchester.
On the other hand … most humans wouldn't be even surprised that the ending of the world turned out that way.
He could imagine his father looking smug right now, because God won the 'love wins, love always wins' argument once more.
Gabriel bet He wasn't even subtle about it, where-ever on earth He remained right now. But at least it meant Gabriel wasn't the only one who had to endure the side effects of archangels lovemaking.
Lucifer had never been particular quiet when he indulged.