Bread is one of those foods that could only be served to guinea pigs as a treat (as opposed to an entire meal every single day). Guinea pigs can overeat just about anything if they put their minds to it, which is why you should always feed them in moderation regarding treats. They can eat bread for sure, but they shouldn’t be munching on the food every single day (and as a main component of their diet). If you’ve been feeding your guinea pig bread on a consistent basis you might want to stop right away, or else it could lead to some very serious complications.
Here is a cute video of a guinea pig eating bread.