The true gospel is one and that is the one Jesus preached bUt we can't find it now what we are using now is historical records which is according to matt. Mk. Luke and John those who compiled the bible are not sure cos they did not receive it direct or has seen their seal, signature or full names, that is why the gospels are written as according to so and so in inverted commas. In these historic books we can see that Jesus went places and preached the gospels and we know that he was given a book from God which he uses to preach, but for sure we know he was not holding Matthew Mark, Luke and John because they were written 40 yrs after he has left the earth. So that he preached is the Gospel.
In these historic records there are things we can count on and that we can't count on, e.g in Mark where he said a true believer can drink poison and speak languages, we can't acertain that. Then we see that really Jesus christ thus said in John 16, that the spirit of truth will come and guide us to all truth which is Mohammad and the Quran which has guidiance to all truth.