Superman would win without a doubt. Whether it's through a plot device, a power up such as a Sun dip or Superman just straight up punching him out depends on the writer.
Not to mention, pre reboot Earth Prime was a protected nexus for the entire multiverse. If Superman can't do it alone, some cosmic being will help him out, like Kismet, The Guardians, Spectre...
Although it's funny how Busiek had Earth outright destroyed, and it all held together, despite what Johns established..
Most likely through a plot device or some crap about "Galactus was touched by the spirit of the young kryptonian fighting for his adopted planet ... blah blah"
Galactus annihilates him. He has so many options to end this fight it's not even funny. A non jobbing Galactus cannot lose unless some cosmic being higher up on the food chain steps in to save Clark.
Galactus can : devour the planet straight up, devour the Sun (there goes the sun dip), suck the energy right out of Clark, transform himself or the Sun into a red sun (now Superman is fugged), transmute him to protoplasm (like he did to Herc in an alternate universe), I can go on but you get the point.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."
Unfortunately, this is true; in the senario put forth by the OP in a comicbook Galactus will job furiously and Superman will over achieve by several orders of magnitude and find a way to defeat Galactus...
Galactus could even be at full power and it wont matter...Galan of Taa will lose.