Galactus sent a probe to check up on the Ka-El. I guess the uniqueness of being the Last Kryptonian appealed to the Big G. That probe was intercepted by Henshaw and modified to make it look like Galactus destroyed Krypton. So when Supes come's the Marvel Universe the probe transports Supes and Reed aboard Galactus's ship and who then gives him the power cosmic.
Of course Supes has no choice at this and is all, "I am a servant of Galactus." But yeah Galctus tricked Supes.