The term "gentleman" is nebulous, and typically means something specific to the one who employs it, while not necessarily meaning the same thing to others who hear it. Context is used to nuance the meaning. For example, the phrase used above, "An old gentleman" would mean to most hearers a quiet elderly man who is kind in his dealings with others. His manner may be quiet different to the male described as "a young gentleman", which for me at least describes a late-teen to early-20's eager but respectful and well-mannered man.
To a woman looking for a mate (or her mother!), "gentleman" might mean someone who holds the door, pays for the meal without question, and doesn't even hint at off-colour humour or sexual innuendo.
To someone from the American South, "gentleman" might mean a person who embodies the "Southern" charm and manners, but a person from another region, though equally gracious, might not earn that description. (When was the last time you heard anyone referred to as a "Northern gentleman"?)
To a culture snob, "gentleman" might mean a relatively wealthy fellow who wears bespoke clothes and drives a European sedan, but who may be a complete jackdonkey to those he considers socially inferior.
I would suggest that, if one is using "gentleman" as a key term, that the context and/or description is provided so that others understand its meaning. Otherwise, one's hearers will apply their own understanding, and communication may be futile.