This really should be the case, BUT that should be the result of alot of Eternity's fights; Eternity is an expert jobber and has been chocked out the Ancient One, harmed by Dormmy, and has been messed over by Nightmare...
Galactus would annihilate any of those 3 beings...
Anyway, in a comicbook, Galactus could very possibly win this, but in a forum fight Galactus only wins if he uses the Ultimate Nullifier...
Thats not exactly true; we dont know just how powerful Galactus can theoretically become...
Afterall, in the Black Celestial Arc, he was going to eat the multiverse; surely that feat would have made him far more powerful than Eternity...
Anyway, I took the OP to mean a very well fed Galactus; in which case he cant win without the UN, but if we are talking theoretical maximums, then Galactus spites Eternity...