There's a whole litany of basic gentleman manners. Everyone is familiar with the majority of them. I don't see the need to go into holding the door for people and saying please and thank you. But there's a good set of more subtle guidelines that I think go a long way in distinguishing a true gentleman from a dude demonstrating good manners to impress his girlfriend's parents.
A good place to start is your mouth. Of course, you shouldn't spit. And if you can help it, you shouldn't curse or swear either. But perhaps more important than that, a gentleman never raises his voice. And it goes beyond keeping your cool in an argument. Any variety of shouting or yelling is unbecoming a gentleman.
I've always found it to be extremely annoying when someone equates louder with funnier as they attempt to joke around. Even shouting across the street to get someone's attention can seem downright vulgar. Imagine a football coach or baseball manager watching the action, drawing up strategy in his mind, then signaling it in. Then imagine the same guy shouting at an referee or umpire or berating one of his players. It's not unual. But it's extremely ungentlemanly. Even in a loud enviroment like a concert. People don't talk during a symphony. They shouldn't converse in the middle of a Kings of Leon concert either.