How would things play out? Would they die a horrible death or would they find a way to stop him etc.
Spectre wouldn't stop him. Galactus is part of cosmic balance he doesn't destroy planets because he is a villain or something he destroys it because it has to happen.
He's invading the DC verse. That "eat a planet, keep Abraxas at bay" crap might fly in the Marvel universe, but things don't work like that in DC. Even if the Spectre decided to just chill and watch (which is moot and doesn't apply in a battle), there's the Phantom Stranger who is just a tier under him, and is more than enough to deal with Big G too.