It's an old rule from before the 20th century which is not really active anymore. While it's still true for old names due to convention (Harry, etc.), new forei

It's an old rule from before the 20th century which is not really active anymore. While it's still true for old names due to convention (Harry, etc.), new foreign words with h- in it are now often rendered with kh- (letter Х). Originally, h- was rendered as g- because at one point, the educated pronunciation of "g" was influenced by Kiev's accent (as it was the capital of Rus) which always spells g- as h-. So the closest representation of h- was "the educated G". However, people outside of Ukraine were later misreading g- in such names as "hard G" because they weren't aware of the original pronunciation, hence H => G for old words.