You're absolutely right. I've never heard that Kenneth is French, or that its meaning has anything to do with "gentleman".
Kenneth was a royal name is Scotland, used by three kings. Other kings and queens of Scotland bore English names (Mary, Margaret, David, James), but none of them had French names.
KENNETH m Scottish, English Pronounced: KEN-eth Anglicized form of both COINNEACH and CINÁED. This name was borne by the Scottish king Kenneth (Cináed) mac Alpin, who united the Scots and Picts in the 9th century. Another famous bearer was KennethGrahame, the Scottish novelist who wrote 'The Wind in the Willows'.
COINNEACH m Scottish Possibly means "handsome" from Gaelic. It is often Anglicized as Kenneth.
CINÁED m Scottish Means "born of fire" in Gaelic. This was the name of the first king of the Scots and Picts (9th century). It is often Anglicized as Kenneth.