A gnat is any of many species of tiny flying insects in the Dipterid suborder Nematocera, especially those in the families Mycetophilidae, Anisopodidae and Sciaridae.
They can be both biting and non-biting.
Most often they fly in large numbers, called clouds. “Gnat” is a loose descriptive category rather than a phylogenetic or another technical term, so there is no scientific consensus on what constitutes a gnat.
If they eat one, it won’t harm them, especially however they hold no nutritional value for guinea pigs and are very hard for guinea pigs to consume.
However, they will irritate guinea pig if they do manage to eat them and they bite a guinea pig.
The only time a piggie may encounter them is if they manage to get into their cage.
So if you have noticed that they have incidentally eaten one, then do monitor your guinea pig for any detrimental effects.
If required take them to a vet for a checkup.
For more foods that guinea pigs can and can’t eat, check out our GUINEA PIG FOOD LIST.