Galan is the only survivor of the previous universe. When the modern universe formed, he absorbed the energy from his reality and was reborn as Galactus. Although Galan died, Galactus didn't realise this would be the first of many times he would meet his demise.
In Silver Surfer #109, The Ultimate Nullifer was used to...well... nullify Galactus' existence. His herald, the Silver Surfer, managed to use the device to return his master to life. During The Infinity Gauntlet saga, Thanos used the Infinity Stones to kill Galactus. When Thanos was defeated, Galactus was revived.
In the 2000 story, The Death of Galactus, the Surfer put an end to the Devourer of Worlds after siphoning his life-force. However, Galactus' death awakened a deadlier being called Abraxas, who had killed dozens of Galacti throughout the multiverse. Galactus was then resurrected in order to undo Abraxas' existence.
During the DC crossover, JLA/Avengers, Krona also destroyed Galactus and turned his armour into a fortress. The next time we saw Galactus perish was 500 years in the future, when his body was turned into an engine to power a planet. When the modern day Galactus discovered that his future-self was being used this way, he killed this version of himself.