Injil names the original revelation given to Jesus. Different classical lexical explanations have been offered, including interpretations connected with an origin or foundation and with something formerly concealed becoming manifest.
Those suggestions are not themselves articles of faith. The central conviction is that God granted guidance to Jesus.
Belief in the divinely revealed Injil and historical study of texts subsequently known by the name “Gospel” are separate enquiries. Conflating them obscures what belief in revelation actually requires.