Typical of contemporary countries in late 17th to early 19th centuries (earlier too, but then we're getting into feudal-like structures which are their own type of complexity), Britain made a big deal out of being a 'gentleman' as opposed to 'common man'. Gentry were eligible for government posts, were commissioned as Army and Navy Officers (whereas a common man would virtually never rise above enlisted or NCO). Gentlemen were exempt from humiliating punishments such as flogging, and overall were considered of entirely different social standing, almost a different citizenship class.

However, when compared to contemporary countries like France or Spain, which had a strict formal nobility system with a clear distinction of being of noble vs common birth, Britain had an extremely limited formal nobility. In Britain (due to stricter inheritance laws if nothing else), only those with formal titles such as Earl or Duke were considered 'nobility', far too few to fill the ranks of 'gentlemen' required to serve in civic and military offices. The idea of 'landless, itinerant noble', the basis of romances such as by Cervantes or Dumas, is not really a thing in Britain. A British peer, by definition, owned land, and would presumably be either busy running it or serving in Parliament (or as a senior General), rather than making a career as some 2nd lieutenant in a non-elite regiment.

That does, however, make the question of 'gentleman' a rather fluid one, compared to a continental fellow, who either was or wasn't a noble by clear-cut virtue of birth. What made a British gentleman? Being born in a manorial household? Having a certain amount of money? Having a university education? Being the younger (or illegitimate) son of a Peer? If someone of 'low birth' self-educated and made a fortune in commerce, would they be entitled to calling themselves a gentleman? What about his own sons?

The definition might have remained fluid (e.g. "you were a gentleman if other gentlemen considered you their peer, and we can leave it at that"), if not for the rather rigorous limitations the title (or lack thereof) conferred on its holder (going back to things like political appointments or military commissions). Surely, with such obstacles, people of means (however acquired) had a vested interest in being recognized as gentry in order to socially advance, and there would need to be some specific criteria of what being a gentlemen actually meant for purposes of such recognition.

As a bonus, if a British gentleman travelled in Europe (e.g. on a Grand Tour, or even trying to serve in a foreign country's military, which wasn't too uncommon at the time), what criteria would those other countries use (like France or Spain, which, as mentioned, had a very strict birth-based rules of who was and wasn't a noble) to determine his social eligibility? Which kind of British visitor would be considered by Continental nobility as 'one of them', and which one wasn't?