What were gentlemen to do when the very nature of "having a profession" meant one was no longer a gentleman?

The technical definitely of a "gentleman" in the Georgian era was a man who did not earn a salary. All money in their purse would have to be inherited or gained by other means than earning a salary. Salary was the kiss of death.

The professions allowable for gentlemen without losing their status were (in order of prestige and acceptance):

• military officer

• clergyman

• barrister (but never solicitor)

• doctor

  
  

Each of these professions were pricey commitments and were not paid salaries, two stipulations for a profession to be acceptable. Anything requiring an apprenticeship or ending in a salary would result in loss of "gentleman" status.

While we use the term "gentleman" now to refer to anyone with good manners, and often refer to Georgian gentlemen as anyone of good social standing, the term was a distinct social class.

The Duke of Handsome, for example, would not be a "gentleman," rather he would be an "aristocrat," while Mr. Darcy would have been of the "gentleman" class.

Gentlemen were those of good birth, high social standing, and wealth, more often than not landowners living from the estate's income, but sons of gentry would have also been considered gentlemen even without owning their own property so long as they never earned a salary and were of good birth, high social standing, and (presumably) the wealth of the family (which may or may not have been true).

For more detail on this subject, enjoy this post by the English Historical Fiction Authors on Gentlemanly Professions.