There is no GSM in South Korea or Japan!
South Korea (and Japan) has no GSM frequencies! Where do you get this information that they do?
If one wants a phone for South Korea (or Japan), one has to compare the UMTS/HSPA/HSPA+ frequencies (all 3 of these acronyms are compatible as far as frequencies go). Comparing GSM frequencies would be pointless, because South Korea has no GSM!