well, posting on ads-l proved helpful.
someone replied to my query about yo motha’s a guntha and made the following suggestion:
that makes complete sense since there’s really no way i’d be able to hear the glottal /g/ sound at the end of thug. so, naturally, it would sound like yo motha’s a GUNTHUH.
any correlations to gunther are non-existent. there were some connections, but they had nothing to do with any such insults. but the only problem is that i can’t see a relationship between gun-thug and an insult to one’s mother.