Be careful here. What you have said is correct, with qualifications. The technical term agraphon refers to something which was said by Jesus but which does not appear in the Gospels. It appears elsewhere in Scripture. "It is more blessed to give than to receive" (Acts 20:35) is the most well-known example. It appears in Acts but not in the Gospels. The quotation is authentic and inspired because it appears in one of the inspired books and is attributed to Christ. Any quotations attributed to Jesus in books not in the canon are of questionable authenticity--in fact, we can't think of any that could be considered definitely authentic.