Pestering... probably; biting... not sure . What have you done already to control them? Other than changing substrate more frequently; not sure what can you use that will kill the gnats without hurting the frogs. You could use yellow sticky traps outside enclosure to try and capture some gnats, or use one of those electronic insect traps to try and reduce their numbers.

Bacillus thuringiensis, an organic control for caterpillars, can be mixed into solution to kill insect larvae. Bt is labeled a safe bacteria to use around people and pets; but usually when writers mention pets they mean mammals. Maybe you can consult a local veterinarian to ensure it's safe with frogs or someone else here might have more knowledge on it. Good luck !