Your question piqued my interest, so I looked this up. The Guilf of Guinea is the name of the part of the Atlantic Ocean stretching from Gabon to Liberia and Guinea seems to have been used colloquially to refer to the entire West Coast of Africa for some time.

Via https://www.reddit.com/r/AskHistorians/comments/3450ws/why_are_so_many_countries_called_guinea/:

That’s how many of the European colonies in the region took on names like French Guinea, Spanish Guinea, Portuguese Guinea, and German Guinea.

• Portuguese Guinea became Guinea-Bissau (named after the city of Bissau).
• French Guinea became just plain Guinea (though it's sometimes called Guinea-Conakry after its capital).
• Spanish Guinea became Equatorial Guinea after its location near the equator.
• German Guinea dropped the Guinea part entirely became Togo and Cameroon, after their historical names.

I believe the Portuguese called that whole area Guinea back in the day. I think it’s also where Papua New Guinea in South America gets its name from; through the tragic migrations of the middle passage.