And now his trainer has revealed the price the actor paid to get ripped, in a regime that has been dubbed the "Southpaw diet".
Former professional boxer, Terry Claybon, 50, said it took six months of training before filming even began, which included six-hours of workouts a day.
"This means muscles will experience long time under tension, triggering protein synthesis and muscle tissue growth, also know as hypertrophy."
Claybon, Gyllenhaal's trainer, told Yahoo: "I wanted Jake to learn how to box so he wouldn’t be out there on the set trying to act like a boxer."
Wanless explained there would have been a big emphasis on "eccentric loading".
He added: "There will be a lot of metabolic stress with this kind of training which causes the release of testosterone and growth hormone.
"The programme indicated training until failure and training to failure causes a lot of muscular damage and can lead to greater muscle development.
"The majority of the exercises were multi-joint compound exercises with some isolation exercises to bring up lagging body parts.
"Rest periods were short, only long enough to maintain good technique and lots of calories would be burnt. Training splits were used meaning the body has time to adapt and recover."
Gyllenhaal had to gain 15 pounds of pure muscle for the film, but his personal trainer said every day he became more eager to learn about the sport.
Claybon said the Southpaw diet involved:
1. Serious exercise - Gyllenhaal had three hours of workouts a day at the beginning, and upped it to three hours of boxing in the morning and three house of strengthening, conditioning, and cardio at night. He did 1,000 sit ups in the morning, and another 1,000 a night.
2. Professional boxers - He got into the ring with professional boxers to learn more about defense, as well as going to watch professional matches.
3. Specific diet - Gyllenhaal stuck to carbs in the morning and protein in the evening.
Sioned Quirke from the British Dietetic Association told HuffPost UK Lifestyle that Gyllenhaal would have been on a diet similar to atheletes or boxers. This would include a lot of meat, fish, vegetables and protein including a lot of chicken and eggs.
However, she advised this is an extremely restrictive diet that the average person shouldn't do.
She said: "Jake was looking to gain a lot of muscle, but he had proper guidance. Any restriction in your diet carries risks because you are limiting your food groups, which makes you at risk of deficiencies.
"We would never encourage the average person to follow this pattern unless they were under strict supervision from a qualified professional.
"Jake only had to do it for a short time, but for somebody looking to make permanent changes to their body, these are not the changes we would suggest."
The actor told Extra after filming the show: "I just trained like a boxer. As you learn the skills, that stuff just happens in your mind and in your body."