Designing the Godbomb
After his past defeat at the hands of Thor the God of Thunder, Gorr sought a different way to kill divine beings other than slaying them one at a time with his own hands, which eventually led him to Shadrak the God of Bombs.
After seeing everyone in his pantheon killed by the God Butcher, Shadrak lost his sanity and begged him to design a bomb capable of killing all deities across existence. After Shadrak designed the bomb, he managed to escape Gorr by hiding in the one place he never thought to look: the cave where he was defeated by Thor. Meanwhile, Gorr set out to build the doomsday device himself.
Building the Godbomb
As he needed the heart of an Elder God, one or two moons and some centuries to build the Godbomb, Gorr traveled to Chronux, a planet inhabited by the Time Gods, to use the Pool of Forevers to travel through the timestream.
After having secured the heart of a misbegotten stripling Elder God, Gorr used the pool once again to travel thousands of years into the future, where he spent the next 900 years slaughtering and enslaving deities in order to build the Godbomb. It took four hundred years for it to be completely built, and upon its near completion, Gorr took the younger version of Thor from the past to the future to witness his crowning accomplishment's fruition.
With the Godbomb complete, Gorr detonated it using the symbiote-sword All-Black, causing every divine being that ever had or would have existed to start dying a slow and agonizing death as the Necrosword ate away at its victims from the inside out. Luckily, Thor managed to absorb both the blast and the Necrosword through the usage of two Mjolnirs, thus canceling out the blast.
The Pool of Forevers built into its core and fueled by the blood of the thousands of gods slain by Gorr enabled the bomb to send a ripple backward through time, exploding through every second and infecting every god across the timestream with a portion of the All-Black symbiote, which also served as the bomb's detonator.