GABRIEL IS NOT DEAD
Listen up, people! THE ARCHANGEL GABRIEL IS NOT DEAD! At least, he's not /staying/ dead.
The reason being? I bloody refuse!
The other reason being? The Shadowling!
As we know, the Shadowling is really picky with the beings in its care; it threw a whole fit about Cas escaping, and about Jack being in heaven.
THEORY ONE
Gabriel died in the Alternate world, but the Alternate!Gabriel died there, too. So the Alternate!Shadowling already /has/ Gabriel, and probably isn't looking to wake up or to take on clutter from a different world entirely. So, the Alt!Shadowling would spit Gabriel out of the Empty, back into the Alternate world. There, Gabriel can take back his vessel and fight to get back to his universe. Assuming the Alt!Shadowling doesn't end up putting him into his own universe to begin with (so the Shadowling doesn't have to keep rejecting the angel every time he dies).
THEORY TWO (which is kinda underwhelming)
The Shadowling from the original universe might get upset that he doesnt get to collect Gabriel as promised, might take Gabriel himself, and either Gabe can rescue himself or Team Free Will can (but let's be honest, if Cas can annoy the Shadowling into rejecting him, just imagine the horrors the Shadowling will face with Gabby).