Gabriel wasn't the only one who left. Another angel has been in hiding. His name, Raguel. Calling for help with a hunt, he ends up getting the Winchesters, and by extension Castiel. Events are set into motion, and the chapters are still being written.

"Plato's Stepchildren, episode 67 of the *original* series, Captain Kirk and Lieutenant Uhura kiss -" Who was on the other end of that phone call Dean made in 4.17 "It's a Terrible Life?"