You must be kidding if you think the Apostles only preached out of the Old Testament. You make the teachings of Christ void by your argument because they were not yet put to writing.
So they can just forget the words of Christ himself:
Mat 28:20 Teaching them to observe all things whatsoever I have commanded you. And behold I am with you all days, even to the consummation of the world.
So now the Gospel is the Old Testament?
But especially see the post-resurrection Jesus talking with the men on the road to Emmaus: Luke 24:13ff
"27 And beginning with Moses and all the Prophets, he interpreted to them in all the Scriptures the things concerning himself. ... 44 Then he said to them, “These are my words that I spoke to you while I was still with you, that everything written about me in the Law of Moses and the Prophets and the Psalms must be fulfilled.” 45 Then he opened their minds to understand the Scriptures..."
The Scriptures are given by God to men who wrote down the words given them by the Holy Spirit throughout our history. It all hangs together. It's all pointing to Christ. It's all about Him. And clearly (seeing Jesus' own actions above), it was necessary to be written, for why would Jesus ever reference it otherwise? He could have simply done a couple of miracles and been done with it.
So yes, the Gospel is the Old Testament, the New testament.... it is the scriptures, it is the Word of God. All. Together.