Gerty Guinea Pig Gerty Guinea Pig is a colourful complete food for cavies. Like humans, guinea pigs cannot generate their own vitamin C and so have to be fed on a specially formulated food. Gerty Guinea Pig is rich in both protein and vitamin C - essential to keep guinea pigs in peak condition. The UK's no.1 guinea pig food contains 11 high quality ingredients to encourage foraging and provide interest to the guinea pig diet.