Here's the difference.
"The author's purpose [in writing Gospel] is thus explicitly theological: to inspire live-giving faith in his readers.
Because the Evangelists [Gospel writers] were committed believers in Jesus' divine Sonship and saw their writing task as more theological than historical, it is particularly difficult for scholars analyzing the Gospels to separate what may be authentic memories of Jesus' words and actions from later theological interpretation (and embellishment) of them."
"Like all literary narratives, the Gospel stories have the basic elements of setting, character, dialogue, plot, style, and rhetorical techniques, such as the use of irony. As readers become familiar with and Evangelist's distinctive views, they will eventually be able to explain why Matthew's rendition of the wedding feast parable, for example, differs from that of Luke of Thomas. In each case, the author edits the parable to fit his religious perspective."
(
Stephen Harris, The New Testament: A Student's Introduction, 4th Edition; McGraw-Hill, 2002; pp 99, 111)
Here's a fair article discussing the Gospel as a genre:
What is a “Gospel” Anyway? A Few Thoughts on Gospel Genre and Why it Matters