A nearly extinct species
of human being
as we move forward into the 21st century
. A gentleman
is defined by his exemplary behavior
towards others, be they women
or even animals
Unfortunately, the idea of being a gentleman has fallen out of fashion because it's often associated with gender stereotyping and immense wealth. But a gentleman is simply a man who behaves respectfully towards others and strives to maintain his own respectability in the eyes of others. A man can be a gentleman no matter how much money he has, what level of society he comes from, or how old he is.
I sometimes use the phrase "Christian gentleman" as a synonym for "gentleman", since they share the goal of respect for others and treating other people as you would have them treat you. But this isn't exclusive to Christianity, nor to religion in general. A gentleman can be of any faith or of no faith at all; he is defined by how he behaves, not why.
A gentleman is, necessarily, male. The female equivalent is a lady. This isn't sexism, it's semantics. The only real difference between a gentleman and a lady (besides the obvious one) is during dancing; the gentleman leads, and the lady follows. And even this can be reversed, so long as it is agreed upon.
Since it's rather difficult to find high school classes in this sort of thing anymore, I've been composing my own "Gentleman's Code" over the past few years.