"The story is about a guy who uses his rage to get great success," Gyllenhaal said at the film's premiere in New York.
"That same rage destroys his whole life. Throughout the film and his journey, he has to learn how to fight - not only in the ring but in life without rage."
Gyllenhaal has said he watched fights and trained intensively for the part.
"(The movie is) very moving. It's got a lot of heart, a lot of grit, it's got a lot of soul in it," she said.
"Boxing is a dog-eat-dog world. There's are a lot of people who are going to be your friends when you're winning and like any business where there's a lot of money involved, you take a long hit and everybody scatters."
Oscar winner Forest Whitaker plays trainer Titus "Tick" Wills, who helps the boxer recover.
Gyllenhaal says he'll keep boxing
Hitting the gym for hours a day to train for the role wasn't the biggest challenge he faced, Gyllenhaal confessed at the premiere.
"I didn't know how to box," he told Variety. "The getting in shape part was the aftermath and the result of learning how to box and learning a language and a fluency. As I got more fluent, my body transformed. I spent months on my jab. I spent months on my footwork."
In the process he discovered he had a passion for the "sweet science." Shooting may have wrapped, but he's still training.
"Boxing will be a part of my life from here on," said Gyllenhaal, adding, "What I was amazed by was the grace and the sensitivity that's needed to be a great fighter. People say, 'Oh, boxing is brutal,' and this and that, and I say, 'You know what else is brutal is ballet.'"
Southpaw opens in Australian cinemas on August 20.