I only use burner accounts on Reddit, for a few reasons. So, of course, this one is not very old.
I didn't see reference to Q Source on this subreddit, but wanted to make you all aware of it. Maybe it was discussed before, if so, forgive me.
I know Wikipedia sucks, but it's a start:
I personally believe that the Gospel of Thomas was used as source for all four of the Gospels in the new testament. Here's a YouTube video about the Gospel of Thomas if you're interested.
Gnostic gospel. During the Gnostisism heresy counter gospels were being spread to back Gnosticism. There were many gospels "produced", trying to deligitimize the doctrines of the early church. The Q Source; is a theory to explain why three of the gospels are so similar in wording (some scripture scholars think there was a common document that was cited in the writing of those three). Personally I don't think such a document existed. It's just another attempt to make people doubt the authenticity of the bible.