Other people will be able to give better answers about how this trope entered the rest of the world, but hopefully I can explain where it comes from within Britain.

The concept of 'gentleman' is a very old one and one which is murky in origin. In it's purest form it simply means someone of the lowest landowning rank, however, it has always had connotations of good conduct and respectability.

Within England/Britain, the gentlemanliness of gentleman has always been a matter of concern. Chaucer touched on it in the Canterbury tales, whilst Shakespeare also discussed it. It is therefore apparent that all through British history there has been a concept of gentility and conduct becoming a gentleman.

In the 19th century, particularly after the Great Reform Act, the concept of class by birth diminished in Britain. Instead of the previous Peer-Squire-Gentry-Commoner social hierarchy based on feudalism, the class system developed. Essentially people became divided into Aristocracy, middle class and labouring classes. Though these were rooted in material differences, a big part of the Victorian class system was based on education and conduct. It was in this period that 'gentleman' becomes divorced from its feudal status meaning and is more generally applied to men of good character.

The Victorians were famously obsessed with propriety and socially sanctioned behaviour. The 'gentleman' became an important construct of hegemonic masculinity, particularly among the middle classes. It was the defining social narrative that men should aspire to be 'gentlemanly'. A vast number of etiquette guides were written about the subject, men were supposed to behave in certain ways, dress in certain ways and generally conform to what was an increasing national stereotype.

This was further developed by the growth of the public (i.e. private) schools. Whereas education was formerly largely religiously based, the needs of industry required that men be more roundly educated, giving rise to new methods of schooling. These schools were aimed at the middle and upper classes, and so sought to instill the values which those classes valued. The schools therefore set out to 'shape' boys into gentlemen - a model most famously set out by headmaster of Rugby School Dr Thomas Arnold.

By the late nineteenth century, the idea of gentility effectively became a second religion for the British. It was a great cultural trope around which education, literature and public life was hung.

As a result, this was how the British came to be pictured. The prominent people in British life were all schooled in this discipline. It was the basis of the Civil Service, military officierships and generally any role in public life. These were the people that non-British people would be exposed to. Furthermore, much of British culture became about being, or mocking, the gentry. Turn of the century film and literature are awash with gentlemen - look at Coward, Waugh and countless others. Even music hall, a working class form of entertainment, relied on the gentry for many of its stock characters.

The view of the English/British as gentlemen largely stems from this period. It was a culture, a national obsession that permeated all British life during the 19th century. It affected the people Britain sent to the rest of the world, and it's own view of itself.

See also: Mason The English Gentleman

Berbreich The Image of the English Gentleman