Did black people ever actually say "gwine," or was that a white misconception?
"Gwine" (just as "de" for "the") is standard in current Jamaican (majority Black population) dialect. Black American dialects have changed a lot over the years. What may have been an honest attempt at description/transcription, at one point in time...once set down in print form, may have been reproduced (guessed at) without finesse or understanding in later (e.g. Amos and Andy) times, and with negative intent.

I grew up in Wisconsin. When I tell people that, they put on what they think is a Wisconsin accent and pronounce it "Wiscansin."
I believe you when you say it, but there has also been going on the famous Northern Cities Vowel Shift in the last many decades (possibly since you left the area?) which is typified by that pronunciation and reflected in what people have heard in recent years -- At least it is what they have heard *different* in Wisconsin (which probably applies just to the areas affected by the shift).

Lots of on-going changes... Which begs the question if singers are also expected to be historians!