Check out Galatians 1:8, 9 where it is quite clear that there is but one Gospel as laid out there. If any other is preached or anyone adds to what has been said, he is to be considered anathema. I am NOT wrong and you may want to read those verses IN CONTEXT.
What is the Gospel, Cronos?
No mention of the Epistles whether by Paul or others -- so then your statement but adding to the written word by adding to the Gospel as presented in Scripture is saying that the Letters of Jude, Paul etc. are adding to the Gospel and wrong?
God's Word is Jesus Christ -- the written word that is the Bible is part of the Word but not the entire Word.
The Gospel as we know them and as I pointed out above are among the only parts of the NT canon that were taken universally as canon from Apostolic times. The letter to the Hebrews, for example, was not..
You said adding to the Gospel while Gal 1:8 talks of preaching another Gospel. There is a difference.
Please read the Bible in entirety and in context.