Gerbils should only be given very small pieces of fruit. In the wild, as a desert species, they simply don't have access to much moisture-rich food such as fruit and veg, and so they have evolved to do without it.
As a result their digestive systems are not well adapted to wet foods, which is why only small pieces should be fed at any one time. A gerbil with an upset stomach is not what you want!
Gerbils like fruit - in moderation
The following fruits are safe for your gerbils. Avoid all citrus fruits, as these are very acidic foods and can cause sore skin in and around their mouths.
Safe fruits for gerbils include (non-exhaustive):
- An apple slice
- A few blueberries
- A slice of banana
- A slice of pear
- A strawberry
- A grape