I'm reading that it helps "delay the glucose absorption in the blood"and that "the basic function of the acids is to bind to the receptor on the intestine, and stop the glucose molecule from binding to the receptor. Thus, gymnemic acids prevent the absorption of excess glucose."
It can also help with "curbing the sugar craving" which could be useful not only for diabetics but also candida sufferers, which, as we know, is connected because of the blood sugar imbalance.
For myself, I'm excited to hear that "when Gymnema leaf extract is administered to a diabetic patient, there is stimulation of the pancreas by virtue of which there is an increase in insulin release." I just might have to look into this further!
"Double-blind studies  confirm that the therapy they have developed can cure up to 60% of patients by restoring proper beta cell function. The deciding factor appears to be whether the antigens which originally caused the auto-immune response which damaged the beta cells are still in the patient's body. Where they remain in the body a cure is less likely because they continue to trigger damaging immune system activity. Work on how to remove the antigens continues." http://www.greenhealthwatch.com/newsstories/newsillnesses/cure-diabetes1.html
And so it would seem that the key is removing the antigens before being able to use the herb successfully.