It has killed quite a few people.
Just read the Wikipedia article on Gerson. Wikipedia isn't the best source, but it's a good start. Gerson may have believed what he was doing, but he declared people cured of cancer just because he or people working for him "felt" that they were. Sometimes they even pronounced people cured over the telephone. When those patients went to regular doctors, it turned out that they still had active cancer and were sicker than when they started. Gerson's medical license was revoked in 1958, and his therapy is illegal in the United States.
My best friend's father-in-law just died of a cancer that could have been cured. He was doing well on chemo; the cancer was almost in remission and he didn't feel very sick. Then some "nutritionist" persuaded him to drop chemo and go on a "curative diet" instead. He got sicker and sicker, but she told him that was just "the toxins" and he'd be better any minute now. By the time his son realized what was going on, it was too late. He was too sick from the diet itself to be able to go back on chemo, and the cancer had been spreading unchecked for months. He actually died from malnutrition and secondary infections, not the cancer itself. I don't know if that diet was the Gerson therapy specifically, but it's very similar and the results were very similar.
Anytime somebody tries to tell you that they have "the truth" and it's been "suppressed by the establishment," hold onto your wallet.