King Kai via better skill, experience, and a wider arsenal of abilities. Earth also has a large amount of life force (helped massively in the Super Spirit Bomb), and King Kai can draw from the sun as a last resort, King Kai also can use the Kaioken. Anyways, while they're in the same tier and have the same speed, Gohan still has a power level lower than King Kai. King Kai is at 4,000. Gohan when Nappa and Vegeta landed on Earth had 981 normally, 2,800 with Masenko. I don't know how power levels are viewed on this wiki, but that should make King Kai significantly stronger than Saiyan Saga Gohan immediately. But even ignoring that, King Kai should still win via better abilities, skill, and experience, Gohan is just a kid with a year of training from Piccolo, and even then, a lot of that training is just him trying to survive in the wild, not to mention King Kai is trying to kill him.
Edit: Also, couldn't King Kai theoretically place weights on Gohan via Weight Manipulation, and Magic Materialization?