Nicknamed ''Iron Man'' for playing in a then-record 188 consecutive NFL games during his Hall of Fame career, Gregg told The Associated Press he's been diagnosed with Parkinson's disease.
Although the cause of the debilitating neurological disorder is unknown, Gregg, his family and his neurologist say his disease may be related to numerous concussions he suffered during his playing career in the 1950s at SMU and from 1956 through 1971 with the Packers and Dallas Cowboys.
The 78-year-old Gregg was diagnosed last month after being referred to Dr. Rajeev Kumar, a Parkinson's expert and medical director of the Colorado Neurological Institute's Movement Disorders Center in Denver.
Gregg's symptoms include hand tremors, a stooped posture, shortened stride and softened voice.
''I'd like to stop it there if I could,'' he told the AP in an interview.
There is no cure for Parkinson's, but a combination of drugs, exercise and physical therapy can delay the devastating effects of the disease that strikes more than 50,000 Americans every year.
The former offensive lineman, who was a six-time All-Pro and nine-time Pro Bowler, said he wanted to go public with his diagnosis to promote understanding of the disease and push for more research. He said he hopes to help others recognize the signs of Parkinson's and seek treatment early enough to delay the degenerative effects of the disease on both mind and body.
''I don't pretend to say that I'm important to the scheme of things in the whole world, but I can do something and help along people who have this disease,'' Gregg said. ''So, I'm kind of just saying that I have it, I want to do something about it and I think I found the right people to help me along the line.
''I had a friend who had Parkinson's and he didn't find out he had it until it was too late to do anything about it and we lost him and when I first heard this, you can believe me, it shook me up.''
More than 5 million people worldwide, including more than 1 million in North America, have Parkinson's, according to the National Institutes of Health. The disease is characterized by increasingly severe tremors and periodically stiff or frozen limbs. Patients gradually lose brain cells that produce dopamine, a chemical key to the circuitry that controls muscle movement. Dopamine-boosting medication and an implanted device called deep brain stimulation can reduce some symptoms.
Muhammad Ali is mostly mute after years of living with Parkinson's, and actor Michael J. Fox also suffers from the disease.
''The thoughts, prayers and support of the Green Bay Packers are with Forrest, his wife, Barbara, and his family as he faces this new challenge in his life,'' said Packers President and CEO Mark Murphy. ''Packers fans have long admired Forrest, from his playing days to recent years when he's returned to Lambeau Field and Wisconsin as a widely respected alum.
''I had the opportunity to get to know Forrest last spring on our Packers Tailgate Tour and know he will use the same inspiring determination that marked his career and previous health challenges to serve him well as he adjusts to living with Parkinson's.''
Gregg beat melanoma in 1976 and colon cancer in 2001.
''This is not my first experience with a life-threatening disease,'' he said. ''So, I've got another battle to fight.''
Gregg first noticed something was wrong while giving a speech in his hometown of Sulphur Springs, Texas, in February.
''Being a football player and a coach, those guys are all loud, and I noticed that my voice had softened considerably,'' Gregg said. ''And I wasn't getting around as well as I had been. I started to stoop over. And I didn't like that.''
Gregg also noticed his left hand trembling. At first he thought maybe these symptoms were just signs of advancing age. His wife and daughter persuaded him to seek medical help.
Gregg's neurologist said the former football player's fitness, lifelong fondness for exercise and fighter's mentality will aid him in his battle.
Kumar stressed that it's uncertain whether Gregg's diagnosis is a result of playing football, although he noted that research shows there is a link between head injuries and neurological disorders later in life.
And Gregg suffered countless concussions during his college and pro seasons.
''One time in college, I went over to the other team's bench,'' Gregg recalled. ''I woke up with an ice pack on the back of my neck and I said, `What's going on?' They said, `You've been gone for a while.' So that's what I know about concussions. That's what I know about getting hit.''
Kumar said Gregg's many concussions may very well have served as a trigger for Parkinson's.
''We know that prior head injury also increases the risk of getting dementia or Alzheimer's disease and increasingly there's a recognition players who have played in the NFL who have had prior head injuries - which is just about everybody - have a substantially increased risk of getting Alzheimer's disease. We don't know if that also applies to Parkinson's, but my guess is that it probably does,'' Kumar said.
Kumar, who said he's also diagnosed another former NFL player with Parkinson's, praised the NFL's crackdown on illegal hits to the head and new protocol on concussions.
Gregg said he would still have chosen to play the sport even if he'd known there would be a price to pay later in life.
''It might have caused me to shorten my career. But I don't know what I would have done differently,'' Gregg said.
A guard and tackle, Gregg is one of three NFL players to win a-half dozen NFL championships, including the first two Super Bowls with the Packers. Gregg finished his career with another Super Bowl title with the Cowboys in 1971.
He went on to coach the Bengals, Browns and Packers, compiling a record of 75-85-1. He led Cincinnati to the Super Bowl after the 1981 season, where the Bengals lost to San Francisco 26-21.
Although his motor symptoms began to show up over the last year or two, Gregg's wife, Barbara, said he began acting out his dreams about 15 years ago. Kumar said this phenomenon, known as REM sleep behavior disorder, was a possible early warning sign of Parkinson's.
One time he dreamed he was trying to strangle a snake and his wife had to sock him to get him to let go of her wrist. Another time he dreamt he was back blocking for Bart Starr and knocked her out of their bed.
''It would be like he was playing football. It was awful. And one time he grabbed my wrist and it was bad. I thought he was going to kill me and I had to say, `Stop, you idiot!' You know, anything I can say to make him stop, and finally I learned to sock him back,'' Barbara Gregg said.
Gregg is on two medications, one to help him with the motor symptoms of Parkinson's and possibly slow the progression of the disease and another drug to treat his abnormal sleep behavior.
Gregg still keeps a busy travel schedule, doing autograph shows and speeches.
Also on his agenda now is speaking out about Parkinson's.
''I think the first thing is probably try to learn as much as you can about the disease. That way if it happens to you, there are no surprises,'' Gregg said. ''I think there are things that can be done to slow this disease down and I think that's one of the biggest things I'm thankful for.''