I can agree with that... and I don't have that problem. Here's the rub... the Gospel is the account of the life, death and resurrection of Christ--definitionally. You seem to think the Gospel includes all of the other things recorded in Scripture as well--all of the Epistles and such. It is not. There is more that was said and done by Christ than what was recorded (John 21:25) because what was recorded was written for a purpose. The Bible is not a full Catechism of faith but rather the story of our fall and redemption. After that, there is still much to say in regards to worship and daily life. The Church has taught these things from the Deposit of Faith given by Christ and the ongoing revelation of the Holy Spirit.
So prove that the apostles taught it just as the Catholic Church does. The assumption of Mary would be a good start since Catholics are obliged to believe it.