A gentleman was originally a man who was 'gentle'- meaning someone who had no need to work. He was unlikely to be gentle in the modern sense because most of them gained their wealth by doing the Kings dirty work- killing a rival or engaging in war on the Kings behalf. Men of action were preferred over the more reflective type- and this bias continues to the modern day with the public school emphasis on physical games.
All in all, they were probably a repellent bunch- brutal, stupid and leeches upon the poor. You need only add 'male feminist' to this list and you would have a complete list of everything I most dislike!
Fortunately the word evolved along with the economic system. The gentry (same root word) evolved a system of ethics to avoid being slaughtered by their serfs as was happening elsewhere in Europe. This involved treating others with respect, and cultivating a specific form of modesty. This was not true modesty but involved establishing status by making great play of being disinterested in it.
Most people who visit the Queen (a sort of female gentleman) are greatly impressed by her modesty and the fact she pours the tea herself. This sort of modesty only impresses when a certain status has already been established. If you were to visit me for tea, you would find me just as modest but would not be nearly as impressed!
The gentleman, therefore must be a person of power- or the modesty would have no effect. Equally, a person assuming the persona of a gentleman assumes an aura of power by doing so.
It is this aura of power, and danger, that differentiates the gentleman from the nice guy. The nice guy signals his low status by treating women well- but the gentleman does the opposite. Gentlemen gained their power by violence and ruthlessness- while nice guys were simply trained to be nice by their lowly role in life.
This is the real reason why nice guys are treated like dirt by women- and why they love the gentleman with a hint of cruelty.
One such fictional creation is John Steed from the Avengers TV series. He is the ruthless one of the group- but hides it through good manners.
Another such character is James Bond. Almost all women prefer the James Bond who treats women the worst- Shaun Connery. In these films Shaun uses women for his sexual pleasure, is quite prepared to kill them. There is even the hint that he raped one of his conquests before she fell for him. Nevertheless he never gets upset or angry at them- this would lower his status in the eyes of his female fans.
I am not suggesting that we treat women the way. Sexual fantasy is poor guide to ethics. Nevertheless we can still learn a great deal. Women are never more than passing pleasures for James- he is intent on saving the world and will drop them in a moment if they get in the way. We cannot imagine a bond girl daring to act crazy with him or make his life difficult- this is because he always has other options.
A gentleman, then, is a powerful man. He is a man who has the strength to take them out of their own craziness while they are with him. He may sexually obsess women because he can never be owned or controlled by them.
In short, he is emotionally self sufficient and a Man Going His Own Way.
Debrett's Guide for the Modern Gentleman