Remember this chick?
If you don’t, she’s Kali. She’s the god of destruction and violence. But she also works with blood spells. She took the Winchesters’ blood to put them on a ‘leash’. With their blood, she could do anything. Kill them, hurt them, bring them back to life…
She didn’t just take their blood.
She took Gabriel’s, too. Kali didn’t die. She still has Gabriel’s blood.
She can bring Gabriel back to life.