What are the unwritten rules about race that Mr. Gilmer tries to get Tom Robinson to break? Does he succeed?

Mr. Gilmer tried to trick Tom at every turn. He suggested that Tom had his eye on Mayella, insinuated that Tom saw himself as superior to Mayella, and even tried to get Tom to accuse Mayella of lying.

"Tried to help her, I says."
"You're a mighty good fellow, it seems--did all this for not one penny?"
"Yes, suh. I felt right sorry for her, she seemed to try more'n the rest of em--"
"You felt sorry for her, you felt sorry for her?" Mr. Gilmer seemed ready to rise to the ceiling.

"She says she asked you to bust up a chiffarobe, is that right?"
"No, suh, it ain't."
"Then you say she's lying, boy?"
"I don't say she's lyin', Mr. Gilmer, I say she's mistaken in her mind."