In christianity|lang=en terms the difference between gospel and epistle
is that
gospel is (christianity) one of the first four books of the new testament, matthew, mark, luke or john while
epistle is (christianity) one of the letters included as a book of the new testament.
As a proper noun gospel
is (christianity).
As a noun epistle is
a letter, or a literary composition in the form of a letter.
As a verb epistle is
(obsolete) to write; to communicate in a letter or by writing.