With aquatic frogs like African clawed frogs, dwarf clawed frogs, and Suriname toads gravel is fine. These are the only frogs one can keep with gravel as a substrate. Anything that is terreatrial, arboreal or semi-aquatic and eats live prey such as insects should never have gravel as a substrate. The risk of impaction is just to great, but in the aquatic enviroment gravel will not stick to food and thus will not be swallowed accidently.