Tom Robinson is gentle because he helps Mayella Ewell and also tries to be nice to her when he spurns her affections.
Tom is described as talking with a “soft husky voice,” indicating that he is not a brutal man. Cal describes Tom Robinson and his family as “clean-living folks.” They go to church, and are good people. Tom Robinson felt sorry for Mayella, alone with all of those kids and basically a kid herself. This is what got him into trouble. He was just trying to help her, but he was a handsome young man and she was attracted to him.
Tom did have a criminal record, but it was not his fault. He was defending himself in a fight. He just didn’t have the money to pay the fine, so he had to serve jail time.
"Got in a fight with another man, he tried to cut me."
"Did he succeed?"
"Yes suh, a little, not enough to hurt. You see, I-" Tom moved his left shoulder. (ch 19)
When Tom testifies, he describes how he wanted to help her, and how he did not push her away when she tried to kiss him. He always tried to act respectful and polite, even though having a white girl throw herself at him was a dangerous position to be in.
The fact that Tom is such a good person makes an even more tragic figure. This is why he is one of the novel's mockingbirds. He is just a gentle and kind man who is targeted by society for no other reason than his race.