Cancer is largely driven by mutations to genes, which can NOT be reverted by the body (once they are in place it doesn't know that they are there anymore). Thus diet plays little to no role in tumor growth/development. The gerson 'therapy' is reliant on people not understanding the underlying causes of cancer. It likely has cost many people their lives.
Diet is where you work to prevent your self from developing cancer. Once you have it however, it will need treated through other means.
Gerson has many documented cases from what I understand. Of course it is all nutrition.
Snake oil. Everything he proposes is false, and will result in loss of life faster than necessary.