I can't add much about gospell singing but here on Guam people seem to be very impressed that I am willing and able to sing without accompaniment. Clearly there is no longer a tradition of singing unaccompanied thanks to 300 years of Spanish influence on the culture. But even the old Chomorro people created musical instruments to accompany their music.

Yet there is evidence that the old timers also sang a capella. One of the UOG professors explained to me about the Chamoritas which were gossip songs sung by (mostly) women while they worked in groups with each woman contributing a verse. Since a large part of Chamorro humor consists of teasing they would gossip about each other, making up "news" to contribute. Friends who've heard a Chamorrita sung by two old women told me the results were inevitably hilarious.