Africa has a nation named Guinea and a bordering nation named Guinea-Bissau. Quite distant from these two countries (but still in Africa) is another country named Equatorial Guinea. Then there’s the big island of New Guinea (partly taken up by the nation of Papua New Guinea) north of Australia. Finally, I’m not sure if they also come from the word “Guinea,” but there are Guyana and French Guiana in South America. What’s the deal?
Because they all lie along the Gulf of Guinea - loosely covering the coastal region of West Africa below the western tip of the continent to the Bight of Benin. As various powers gobbled up various areas they became local “Guineas”. The country of Guinea was French Guinea, the country of Guinea-Bissau was Portuguese Guinea, Equatorial Guinea was Spanish Guinea. New Guinea comes from the melanesian inhabitants superficially resembling black Africans. The Guinea coin comes from the fact that most of the gold originally used to make them came from the general region of Guinea, etc.
ETA: Well, obviously only the African ones are on the Gulf of Guinea :). What I meant is they all derive from the name for that general region, which on some wiki poking apparently derives from a Berber word for “black”, as filtered through Portuguese.
Only Equatorial Guinea is on the Gulf of Guinea – Guinea and Guinea-Bissau are a bit further north. However, Wikipedia’s article on the Gulf of Guinea suggests that you’re right about their names coming from the region’s name.