What is the meaning of the Gospel's origin being sound, and how can we be certain of the authenticity of texts within it whose original transmission is unknown?

The Gospel (Injil) originated from God and was revealed to Jesus. Belief in it is one of the pillars of faith. However, alteration and distortion occurred within it, as the Gospels currently in existence were authored by four men and are not the word of God. Nevertheless, there are texts that were not distorted during the time of the Prophet Muhammad, peace be upon him. Therefore, what is proven to be distorted, we affirm its distortion. As for what is not proven, we entrust its knowledge to Allah Almighty, so we neither believe nor disbelieve its adherents.