What does Atticus Finch do to define himself as a gentleman in Lee's To Kill a Mockingbird?

Atticus Finch defines himself as a gentleman by means of his speech and actions throughout the narrative of To Kill a Mockingbird.

• Atticus is always polite.

--Atticus acts in a gentlemanly manner at all times as he speaks politely to all the citizens of Maycomb regardless of what they say to him or about him. 

For instance, despite the insults and derogation of Mrs. Dubose, Atticus tips his hat to her and "waves gallantly to her." Then he addresses her, "Mrs. Dubose! You look like a picture this evening."

After Jem cleans up the destruction of Mrs. Dubose's camellias, an act done because of his anger over her insults, he returns home and tells his father that he has apologized and offered to work every Saturday to try to restore the flowers. Atticus scolds him for saying he is sorry if he is really not; further, he explains to Jem,

"...she's old and ill. You can't hold her responsible for what she says and does."

--During the trial, when Bob and Mayella Ewell give their false testimonies, Atticus is polite despite the fact that he knows that they are immoral people unconcerned about their intent to destroy Tom's life and are willing to commit perjury to do so. In his interrogation of these witnesses, Atticus is polite and professional, addressing Mayella as "Ma'am" and phrasing his questions to both Mr. Ewell and his daughter in a temperate tone. In fact, when he questions Mayella, his politeness actually elicits an answer that she probably would not have given him otherwise. For, when he questions Mayella about her father's behavior towards her, Atticus is able to obtain her affirmation of his mistreatment:

"Except when he's drinking?" asked Atticus so gently that Mayella nodded.

--Atticus never passes judgment upon people.

When the children create scenarios involving Boo Radley, Atticus scolds them:

"I'm going to tell you something and tell you one time: Stop tormenting that man." (Chapter 5)

--Atticus never takes advantage of people or other creatures; he is "civilized in his heart," as Miss Maudie says.

After Jem witnesses how well his father shoots a rifle, he understands why Atticus has never hunted. Miss Maudie explains that God "had given him an unfair advantage over most living things," so he has not felt it fair for him to hunt.

Also, when others contradict him or mistakenly believe things which Atticus could exploit, he never judges them; instead, he merely asks them, "Do you really think so?" 

--Atticus is firm in his own convictions, yet he is open-minded.

This firmness of conviction in Atticus is demonstrated in several incidents that occur in the novel. Perhaps the most illustrative example is in Chapter 30, as Sheriff Tate offers to protect both Boo Radley and Jem from suspicion regarding the death of Bob Ewell. However, because of his ethics, Atticus objects:

"If this thing's hushed up it'll be a simple denial to Jem of the way I've tried to raise him....Before Jem looks at anyone else he looks at me, and I've tried to live so I can look squarely back at him...if I connived at something like this, frankly I couldn't meet his eye, and the day I can't do that I'll know I've lost him....I can't live one way in town and another way in my home."

Tate pulls out a knife from his pocket as he tells Atticus that Jem was flung near a tree, and Bob Ewell stumbled over a tree root, falling. He demonstrates how Ewell landed upon the knife he was holding; the blade went into the "soft stuff" between his ribs, killing him. The sheriff contends that Ewell fell on his own knife and died; Jem did not stab him.

Listening, Atticus simply believes that Sheriff Tate is still trying to protect Jem, but Tate argues that with a broken arm, Jem could not tackle and kill a grown man. Further, he tells Atticus,

"It ain't your decision, Mr. Finch....For once, if you don't see it my way, there's not much you can do about it....There's a black boy dead for no reason, and the man responsible for it's dead. Let the dead bury the dead this time, Mr. Finch."

Finally, Sheriff Tate convinces Atticus that it is no crime for a citizen to do his utmost to prevent a crime, and this is precisely what Boo Radley has done. Tate feels, also, that there is no need to drag Arthur into the limelight and cause him trauma. "If it was any other man, it'd be different. But not this man, Mr. Finch."

Atticus Finch comprehends the words and intent of Sheriff Tate, and he concurs with Scout, who surmises that arresting Boo would be "sort of like shootin' a mockingbird, wouldn't it?" Then, Atticus turns to Boo Radley, "Thank you for my children, Arthur."