A gentleman is an historical term used in the British class system for an educated man who is wealthy. Its significance is that in a class system, people were educated, lived and worked in quite different ways according to the circumstances of their birth. This is behind the otherwise strange divide in sports like cricket between amateur and professionals (who often played in the same side), and between Rugby Union and Rugby League.
This distinction is needed to understand many of the novels, plays, and films of the pre-WWII era. Similar distinctions ran in most other countries before the modern era. Consider the characters in Tolstoy's War and Peace as an example.