I guess Brook kingdom was the previous kingdom from Manmeyer family
Goa Kingdom gonna be the kingdom the Nerona family ruled in the past
Could be that too, but it seems that Gunko was assimilated into the family, she doesn't seem to have blood ties. Why wouldn't Gunko go to the Nerona Family then? Maybe because they're "extinct" now (there's no solid evidence so far ofc, but there are some theories out there).
The Manmeyer's could even be a branch of the Nerona's... it's actually pretty common in real life. For example, the House of Bourbon, who are the monarchs of Spain since the 18th century, is a branch of the House of Capet/House of France, who ruled France for a long time until the monarchy ended (with the French Revolution).