Little is detailed about Galactus' life before he arrived to consume Earth of Earth-2149, which was infected by the zombie plague; however it can be presumed that it followed much the same path as Galactus from Earth-616.
Galactus being eaten by zombies
When Galactus arrived on Earth, he paused before feeding and asked the Marvel Zombies about the whereabouts of his herald and was shocked to discover the Silver Surfer had recently been consumed by the undead heroes, who then acquired portions of his Power Cosmic. In his rage, Galactus battled the zombies, who in turn realized they might stand a chance due to their Power Cosmic. Despite their new powers, the zombies were no match against the more powerful Galactus, and they were forced to retreat while he prepared to devour the planet. Later on, when the zombies returned to fight Galactus again, they saw him fighting off many other zombies. Using this as a distraction, they augmented their Power Cosmic with the help of some advanced technology, and manage to stagger and ultimately knock Galactus unconscious and consume Galactus, thus absorbing his powers and becoming his successors, later traveling to different planets for sustenance.[1]
Trivia
After the zombies eat his flesh, they had absorbed his cosmic powers. However, his Earth-Zcounterpart was eaten too, but it appears his flesh did not give them any cosmic powers.
You dare--threaten--the world--devourer--You will--you--will come to regret your actions. NO MORTAL INSECTS CAN THREATEN GALACTUS! 
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