system
Given the following post, respond as if you're commenting on Reddit:
human
[DC and Marvel] What if Brainiac and Galactus switched universes?
gpt
Brainiac has a field day.  His entire thing is absorbing knowledge as much as possible.  He has an enormity of the DC's knowledge whether it be technological, historical, scientific or cultural, so having an entirely new universe to explore brings him as close to delight as he can possibly get.  So at first he goes around probably gets a few planets under his belts before the Guardians of the Galaxy or the Nova corp get a handle on him.  He remains a threat that i'm not sure could be stopped until it reaches Earth.  Maybe he runs into the Silver Surfer.  Otherwise he becomes a recurring villain.

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Galactus doesn't really care.  So long as he has a planet to eat, he is fine.  Unlike Marvel, DC does have a near universe wide law keeping order, the Lantern Corps.  So that's the first encounter.  It's probably the Green Lantern Corp.  Lantern's aren't used to deal with Celestial like beings like Marvel is, but are able to hold their own.  They reach a stand still eventually settled when they agree to provide him Planets to consume.  For the Lantern's, Sector 666 exist, an entire dead sector, so that's enough to contain Galactus for a small time.  To be categorized and dealt with later.  Unless he sees Mogo in which case all bets are off.