Many people are asking this question. So I shall answer them all in this post.

What CAN your GP eat and what they CANNOT eat.

Can / Should eat more often:

Red or Green Bell Peppers: One slice of a whole pepper, given daily, remove seeds. Very high in vitamin C, all colors have high vitamin C content. Please note: Bell peppers are not to be confused with red hot chilli peppers which are a totally different food and should never be given to guinea pigs.
Broccoli: Half a floret, including the stalk, twice weekly.
Kale: Two to three small leaves, twice weekly.
Carrot: One baby carrot or a small slice of a large carrot, every other day.
Romaine Lettuce: One large leaf, every other day. Never give guinea pigs iceberg lettuce, its not nutritious and it can give them an upset tummy and diarrhea.
Cucumber: Very little nutritional value, but has high water content. One slice, include the outer layer which is their favourite part. Given every other day. Cucumber is really appreciated by guinea pigs in hot weather, it acts as a liquid and is nice and cool.
Parsley: A few sprigs , given weekly. Very high in calcium so should be limited if your guinea pig is prone to developing bladder stones.
Apple: One small slice, include peel, give weekly, remove core and pips.Many fruits are full of natural sugar and have fruit acid. Give as an occasional treat because of the high sugar content.
Wheat Grass: Personal favourite! Has good Vitamin C, Juicy, Guinea pig loves them!

Quoted mostly from: http://jackiesguineapiggies.com/fruitandveg.html
CANNOT / SHOULD NOT BE GIVEN AT ALL!

Iceberg lettuce – It’s been known to give guinea pigs diarrhea.
Any type of cabbage – like iceberg lettuce, a small piece is not likely to severely harm or kill your guinea pig, but it’s very gassy and not good for their little systems. Stay away from it if you can.
Cauliflower – also very gas-producing.
Potato peelings – not that this would be something you’d be likely to give your pigs anyway, but toxins can exist in potato skins, particularly if they’ve begun to turn green, and they can be poisonous.
Raw beans (This means things like raw, hard kidney beans, split peas, pinto beans, etc.)
Any type of meat or fish – Guinea pigs are vegetarian and love a salad over a steak any day.
Dairy Products – while many stores will offer ‘yogurt drops’ and other ‘treats’, dairy is not healthy to feed your guinea pig and can actually cause problems. Feed natural treats like veggies instead of store brand treats. It’s less expensive and your pigs will be much happier.
Chocolate, coffee or other caffeine-containing products.
Alcohol (this should go without saying!!)
Cookies/bread.
Ice cream.
Any processed or fried foods that you would eat, including potato chips/nachos or french fries.
Cheese.
anything spicy. jalapeno peppers, anything with cinnamon, etc.

Quoted mostly from: http://www.squidoo.com/guinea-pig-care-4

This should answer most question.