how to make a gospel letter?

Question

dear _________

Answer 1

The word "Gospel" means "Good News". To write a "gospel" letter would simply mean that you are putting in your letter good news.

As a side note: The 4 Gospels in the Bible (as all the New Testament books ) were written by eye-witnesses to the Christ.
Their "Good News" is that the Christ became flesh and brought us the hope of the resurrection.

Answer 2

Maybe something like this:

Dear beloved Brother and Sister in Christ,

Answer 3

seek God,ya know He promised to give us words to speak