Grapes are safe for guinea pigs to eat but because of the high sugar content (over 15% sugars), this fruit should be considered a rare treat. They can eat the skins too but you should remove any seeds or give the seedless variety.
Both green (or white) and red (or black) grapes are very high in sugar which is something that is not good for your cavy. Too much sugar can lead to obesity and other health problems in your pet. If your guinea pig is overweight, it is best to keep sugary fruits to a minimum, focusing on vegetables and herbs as well as safe, freshly foraged weeds.
Always give grapes a thorough rinse to remove any chemicals or bacteria that might be lurking and remove them from the vine, taking off any stalks that remain.
Any fruit should only be fed to your guinea pigs on a very occasional basis. To keep your pet healthy, grapes must definitely not form a main part of their daily diet, but thought of as more of a treat.
Below is the nutritional value per 100g of grapes