Glycaemic index is a term used to measure glycaemic response --that is how quickly a food item gets converted into sugar and enters the blood stream in comparison to an equivalent amount of sugar.

GI and Diabetes
GI of glucose is taken to be 100. The closer the GI of different foods to glucose, the more unsafe it is for a diabetic.
A high GI value is 70 or more
A medium GI value is 56-69
A low GI value is less than 55

If you study different glycaemic index tables available on the net, you will be confused to find varying GI values for the same foods. This is because of different methods of testing, portion sizes used and also because the carbohydrate, protein and fat content of the same food may vary from place to place. For example the amylase content of white rice varies so much that the glycaemic index is depicted anywhere from 90-100.

Rice and Diabetes
Brown rice (GI around 48) has the highest amount of fibre; it also contains selenium that prevents aging, omega-3 fatty acids that are good for heart and brain, vitamin E that is good for skin and blood vessels, and vitamin B-1 that is good for nerves.

Parboiled rice (GI around 67) is rice that is boiled to 70 degrees and quickly cooled. This pushes all the useful nutrients into the rice from the bran before its removal. So par boiled rice is nutrient rich but lacks fibre.

Basmati rice (GI around 65-70) lacks nutrients but has a lower GI than white rice with the best flavour.

Polished white rice (GI around 90-100), has no nutrients, and no fibre.

Bananas, mangoes, grapes and Diabetes
Slightly unripe and smaller bananas have lower GI (around 32) -- can be eaten. Riper and larger ones have higher GI (54-70)
Eat small helpings of mangoes (GI 50--- fully ripe around 70), grapes (GI 48-50).

Some high GI foods—(avoid)---Sugar, sweets, cakes, pastries, honey, gur, sweetened juices, dates, white rice, rice-flakes, maida, white bread, sabudana, Arrowroot, Potato, sweet- potato, and other underground vegetables, chickoos, sitaphal.

Some low GI foods-Sprouts, whole pulses, legumes, salads, boiled egg white, chana, whole wheat and bajra, besan, isabgol, rajma, peanuts, kabuli chana, brown rice, low fat milk, low fat curds, apples, pears, berries, and citrus fruits.

Once in a while when you eat a high GI food, combine it with a large portion of low GI one to balance out your blood sugar. GI is also important to the overweight because if sugar enters the blood stream rapidly it cannot be utilised fully for energy and the body converts it into fat for storage.