If it's dual band it should work in Japan. I called T-Mobile that sold the Samsung Gravity2. The answer varied, no it doesn;t, to maybe it does.

I wouldn't put it past a vendor to give specs in a way that
can be misinterpreted to mean that the phone is more capable than the phone really is, and yet maintain deniability of an attempt to mislead.

But I did not trust the T-mobile people, as T-mobile calls a both, a triband GSM and a quad band GSM and a UMT world phone. T-mobile directed me to an outfit that rents phones, if wanted to have a phone that worked in Japan as if the question should be posed to someone other than the vendor that sells the phone.

Samsung says it's a dual band AWS/IMT.

Congratulations! you were right.