Garp was named the Hero of the Marines POST God Valley.
God Valley IS where Garp's reputation went from "Crazy Vice Admiral who hates the CDs and Roger" to, "Garp the Fist, Hero of the Marines."
Yes, Kong is as Admiral, and he shuts Garp down all the time, like when Garp wanted to recruit the Giants of Elbaph.
Garp DID NOT have political power before God Valley. He had NO ability to call someone up and demand Dragon's location.
Kong wouldn't tell Garp because his bosses are the Celestial Dragons and they DO NOT want Garp near them.
God Valley is where Garp got his reputation. Post God Valley Garp might be able to get away with demanding Marine locations, but, before this, he had absolutely no political power within the Marines/WG.