I think there’s no great difference between “genteel” and “gentle”. The first maybe is sometimes used ironically and has some additional meanings as: “stylish”, “modish”, “too mannered”, but both must be derived from the same Lat. gentile(m) (that is from gente(m) “tribe”, “people”, “breed” etc.). The French “gentil” (as well as Italian "gentile") is the same. Probably “Gentleman” is a gallicism, loan translation (calque) from “gentilhomme” (the use of the latter isn’t as frequent in French though).
That’s very interesting that English seems to have two corresponding words for one they’ve bred from whereas French and Italian have just one. That’s a simple example showing that English vocabulary is much more rich and complicated, loaded with synonyms or similarities in the meaning. Every language tends to borrow words from another, but if other languages do it simply by borrowing or making calques, English develops more than one lexical unit from each foreign word. So learning of English requires our redoubbled efforts. Can we ever learn it and survive?[/b]