The Gamble Era covers Season 6 and Season 7, when Sera Gamble took over showrunner duties from Eric Kripke. The two seasons introduce loosely-related arcs as opposed to the Kripke Era, but both deal with the power vacuum left by the averted Apocalypse, and the collective rise of monsters to prominence. The former season begins a year from the aftermath of the Apocalypse, but also explains what happened in the interim. It featured a three-way power struggle between Heaven, Hell, and a new major supernatural dimension, Purgatory. The latter introduces a drastic change in the balance of power as Castiel declares himself the new God, and with the arrival of new powerful beings referred to as the Old Ones from Purgatory.
Heaven and Hell are both embroiled in civil wars of their own. Belligerents of Heaven are the archangel Raphael, who wants to free his elder brothers to restart the Apocalypse, and the Seraph Castiel, who wants to retain the status quo and promote free will. The demons of Hell are split between its new King, then only King-of-Crossroads, Crowley, and the daughter of Azazel, who led the Lucifer loyalists.
The season also focused on souls, and was a major plot device for Sam Winchester's return from Lucifer's Cage, and for the struggles of the three supernatural planes. Eve, matriarch of Purgatory was drawn into the war and amassed an army of her own to prevent Crowley and Castiel from taking the souls in Purgatory to win their respective civil wars. The stand-off resulted in the death of Eve, and with Castiel betraying Crowley, the angel denying the demon of half the souls of Purgatory. Castiel killed Raphael and declared himself as the new God.
This season provides more depth to Purgatory lore as it introduces its oldest natives, referenced in the previous season as the Old Ones; the Leviathan. The Leviathan were God's first creations, predating angels, human and the soul. They are indestructible, intelligent, cunning, and were locked up in Purgatory for such reasons. They were absorbed by Castiel from Purgatory. The Leviathan soon overpowered Castiel and escaped his vessel. They took control of much of the economy and plotted to harvest humans as cattle, and at the same time, planned to eliminate competition from other beings, such as other monsters, and even demons. The Leviathans managed to bring the Winchesters on the run, and successfully killed Bobby Singer. The season ended with the Leviathan threat neutralized by the death of leader Dick Roman, and Dean and Castiel transported to Purgatory, Crowley taking captive of Meg, and the newly ascended prophet Kevin Tran, leaving Sam Winchester by himself.