A knight is a title, which requires strict training and cultivation, is awarded by the ruler, and is not a folk product. A knight lives at the bottom of the upper class in Europe. He has a fief, lives in a castle, and often takes orders from official.
Knights believe in a faith. Standing on the solitary peak, letting the wind blow around the torn clothes, patting the dusty armor, raising the blood-stained face, smiling, and staring into the distance. Honor, etiquette, humility, perseverance, loyalty, pride, piety...
Gentleman is more like a code of conduct. Be gentle, courteous, well-dressed, and speak elegantly. If you do this, you can be a gentleman even if you are a commoner. It is said that the gentlemen in England even have instructions on where to go around the dance floor to send a partner to the bathroom, and where to wait. The most important thing is that gentlemen are very popular with ladies because they have four words engraved on their foreheads: respect women.