Too much American food, maybe? Did he start late in life?
No, actually he started learning taiji in 1936 at the age of 16. Back then as he put it, taiji was very much looked down on by your average Chinese youth, who gave taiji the derisive nickname of "old men fishing" (as in "grabbing at the water trying to catch fish") Gabriel and his brother Peter were the youngest students of his teacher Liu, who had learned his taiji directly under Yang Ben Hou. Virtually all of the other taiji students were in their upper 50's and above.
Gabriel's physical problems started off when he was in the Kuomintang throughout WWII and later during the Long March. He caught TB on the Long March and carried it around with him until the KMT made it to Taiwan, where he ended up eventually having a lobectomy in the military hospital there. After that he'd be in vigorous health except for the fact that every 6-8 years or so he'd come down with pneumonia for a week or so.
As I said, in between pneumonias his taiji kept him incredibly strong and healthy. I remember when he was in his mid-50's and a group of us students were helping him move out of his house and into a house across the street, he wanted to take a refrigerator from the basement. Two of us students took the top while Gabriel took the bottom. Carrying the refrigerator up the stairs, Gabriel was on the bottom and practically pushing us up the stairs, showing virtually no strain at all (and the bottom of the fridge was of course where the motor was and everything). Into his 60's he was still doing one-legged "pistol" squats all the way down and up.
He made his living as a gourmet Chinese chef, having learnt how to cook from his mother. He knew well over 300 Mandarin recipes by heart and would cook everything from scratch, going into peoples' houses and making up these 12 course Chinese meals for anywhere from 8 to over 100 people. For one Thanksgiving meal he'd made for us students, he served a variety of "beef", "chicken" and "fish" dishes, all made from the same 20 pound turkey-- but the spicing he did made the dishes actually taste like beef, chicken and fish with the appropriate textures.
But I guess in the long run the medical effects of the war finally caught up with him regardless of everything else.
He ended up doing a lot better than his brother Peter, who Gabriel describes as actually having learned taiji to a far greater degree of mastery than he did (Peter, according to Gabriel, could for example jump off a diving board and while up in the air kick his lower forehead with both feet--totally straight legs-- before going for the dive). Peter ended up staying in the People's Republic after the war. During the Cultural Revolution in 1966, he was arrested and made to pull a one-man plow in the fields for the next three years. After that experience according to Gabriel, Peter was "broken" and had lost all his taiji. I saw Peter once in the mid-1980's when he was once allowed to come visit Gabriel in Ann Arbor. He was extremely nice, but frail by that time.