In Africa you have a bunch of Guineas (Guinea, Guinea Bissau, Equatorial Guinea) and in South America you have/had a bunch of Guianas (French, Dutch and British Guiana).
What's the reason behind the names? Always thought it related to location, but there's also Papua New Guinea in the East.
For "Guinea", the actual etymology is uncertain, but a Tuareg origin is the most likely.
From the OED concerning the etymology:
The geographical name appears first in Portuguese as Guiné (hence Spanish Guiné, French Guinée); its origin is unknown
From Etymonline:
presumably from an African word (perhaps Tuareg aginaw "black people"). As a derogatory term for "an Italian" (1896) it is from Guinea Negro (1740s) "black person, person of mixed ancestry;" applied to Italians probably because of their dark complexions relative to northern Europeans, and after 1911 it was occasionally applied to Hispanics and Pacific Islanders as well. New Guinea was so named 1546 by Spanish explorer Inigo Ortiz de Retes in reference to the natives' dark skin and tightly curled hair. The Guinea hen (1570s) is a domestic fowl imported from there.
For Guiana, it's an unrelated word that just happens to sound similar. From Oxford Dictionaries (not the same thing as the OED):
From an American Indian word meaning 'land of waters'.
The problem is I can't tell you off hand what language it's coming from, but it should be Arawak. Etymonline gives a different definition, "respectable", and neither of these are well sourced. I haven't had a chance to check an Arawak dictionary to confirm either of these potential meanings.
Regardless, it's almost certainly not cognate with Guinea. The sound correspondences don't line up as you'd expect them to if they were related.