*Disclaimer* This MyTake will categorize the male gender. This, however, is not to say that males only fall into these categories. These categories are simply the largest types of males out there, and represent most males in a general since.
We all know many children in our life. All boys were once at this level of social maturity, and some have not yet grown out of it. The characteristics of a child are the opposite to that of a gentleman: they are disinterested, they view others as objects (usually as "toys" in the more innocent of definitions), and they are shy and lack confidence.
Some guys are just not interested in a relationship, for many varying reasons: they are focusing on school or work, they are committing themselves to a life goal, etc. These guys are probably the most well-rounded of the group that is the Guys, but relationships with them are guaranteed to lack romance. Usually, all they need to do is become interested in romance, and violà, they are gentlemen.
These guys view others as objects. They are brimming with over-confidence that is the result of burying their insecurities about themselves. They are usually very vain, taking an unusual amount of pride in their looks, their possessions, and/or their intellect. In all of their relationships, they are distached emotionally and fleeting.
Nice Guys are guys who, at their core, are good people, but they haven't matured socially. Some Nice Guys act the way they do because they think that it's what women want. Nice Guys lack confidence, and often dislike themselves: the way they look, their possessions, and/or their intellect. In all of their relationships, they put far too much emphasis on their significant other, to the point that they smother them with attention.
The pinnacle of manhood, gentlemen embody the best a male can be. Gentlemen take interest in their relationships, valuing the other and themselves within the relationship. They are well rounded individuals who have their life in order, and don't require anything from those they love besides their love. They view their significant other as an equal, a partner on the path of life. They are not afraid of the future, and they always respect the decisions and views of the other, but is also willing to make his own decisions and have his own views. His relationships are strong, held together by mutual respect for the other.