Here's why:
* "Spell" in "gospel" is derived from the Old English "godspell," meaning "good news" or "good tidings." It's a compound word from "god" (meaning good) and "spell" (meaning news or story).
* The "spell" in "gospel" doesn't refer to the magical sense of the word. It's a different root than the one related to witchcraft or magic.
So, the best definition of "spell" in "gospel" is "news" or "story."