One year.
One year was all that it took for their plan to reach its conclusion. The vast majority of the human race had been transformed into massively overweight, three-headed canine women. Sure, there were still some human holdouts, survivors that were scattered and isolated, but those dwindled by the day as packs of hungry and lustful Cerberuses hunted them down to share their gift, and the gods greatly enjoyed watching the fruits of their labours.
But so engrossed were they, watching as the surviving humans were caught and exposed to the wild carnality of their pursuers, the process of which had them growing fur, sprouting an extra pair of heads, bursting out of their clothes as their bodies swelled with flab, their reproductive organs morphing into that of the female sex... that the gods didn't notice a flaw in their plan.
At least they didn't, at first.
Years and time mean little to immortal entities, a year can pass in the blink of an eye for them, but in a single year, many things can happen in the mortal realm. Like societies becoming completely matriarchal, technologies and structures being torn down and rebuilt or renovated in favour of simpler and larger designs to accommodate the packs of overgrown canine women... and of course, existing religions were being altered and beliefs changed to match the mindsets and physical attributes of the people, for if Humanity had been made in the god's image, then surely the new Cerberikind had also been molded and reshaped from similar gods.
And so, some gods began to realize that they too were not immune to the 'virus' they had unleashed. It started with one goddess of fertility, she felt oddly empowered, stronger, but otherwise remained oblivious to the physical changes her form was undergoing: her regal attire shrunk, the fabric receding to expose her curves and generous proportions, until the goddess was clad in nothing but a scanty bikini; said curves, already soft and inviting, suddenly began to inflate with new fat, as she grew into an absolute behemoth of a woman, even dwarfing the Cerberus mortals walking the Earth in mass and height, to the point that she towered above the imposing god of war, her husband, standing beside her; her head was soon joined by two others at her sides, as her beautiful and graceful facial features stretched into bloated canine muzzles.
The few gods that were aware of this stared slack-jawed at her, even as her gut reached well past her knees, and her bosom swelled to sizes never before seen by mortal and god alike.
Next to her, the god of war developed similar proportions, as he became increasingly less rugged and more feminine, the ornate armor molding into something skimpier, befitting of her new boobs and attributes, eventually matching her wife in height and headcount. The two proceeded to cuddle, their multiple heads whispering, giggling and kissing each other lovingly. Similarly, an elderly-looking god sporting a long beard changed too, said beard shortening and disappearing as the god regained his youth, his body swelling and taking on more feminine traits until a new goddess of beauty and wisdom took his place and the former's robes having vanished completely, leaving her in the nude, like several of the other new and growing goddesses.
Of the few gods that were aware, some resigned themselves their fates, embracing their new three-headed swollen forms and enjoying the plushness and intimacy with the others, while one or two panicked and tried to weave a spell, something, anything, that could stop the change or at least spare them.
It was all for naught.
Their incantations ceased the moment they felt both their voices and their figures becoming softer, staring blankly into space as new thoughts and feelings took root and buried their fear and panic. It was only when their bellies and breasts squished into each others' that their collective heads cleared up and they joined the other goddesses in revelry, conjuring ambrosia and delectable treats from nothing and happily glutting themselves whilst watching the dawn of a new world, a new pantheon of multiple goddesses representing fertility, beauty, wisdom, harvests, and love.
And their first act together?
To bless the world with plenty of bounty, so their worshipper's bellies wouldn't ever go hungry...