I agree with you, but do not quite understand what it means to be a Gnostic work. If you mean the teacher Mani and his likes, as well as some sects today mixing Christian teachings with that of other religions, then I agree that such an understanding of the term 'gnostic' is not acceptable. Perhaps because of these sects we developed such a bad thinking about the term 'gnostic'. But there is also an Orthodox understanding of the term.
I am sure that St. Clement of Alexandria is an accepted Orthodox father. According to St. Clement, a true Gnostic is one who enquires the truth deeply compared with an average believer. http://www.copticchurch.net/topics/patrology/schoolofalex/IV-StClement/chapter2.html
"Book 7: The Christian Gnostic: It is a defense and glorification of the Gnostic Christian. He alone is the true worshipper and the real philosopher, who grows up to become in the likeness of God. The pagans made their gods in their likeness.. .He attacks the anthropomorphic gods of the Greeks and defends the true Gnostic against charges of atheism and impiety. He then passes to a positive evaluation of the true Gnostic, a laborer in god’s vineyard, who gives help to all in need. He attacks various heretics as foolish, and ends with an account of the Stromata."
When we know for what reason some work is not accepted, we know what to reject in it. We can find truth spread in different writings. But the Church may declare a work as non canonical to help all believers - both those who enquire deeply and the average believer who is not involved in any enquiry.
Why the Gospel of Thomas reached only the school of Alexandria? If my belief is correct, St. Pantaenus visited India (as a representative of the Alexandrian school) and brought to Alexandria the works from India. The Pachomian monks hid the works among the rocks.
It is understanding (unity in mind and thought as Apostle Paul puts it) that is important. We find elements of truth in many works. But the important thing is that we are not asked to follow any work, but only those according to the accepted teaching of the Church.
Right now I am interested in finding a specific truth in any work (canonical, or not canonical - because truth is truth whether it is in canonical work or non-canonical work.) - i.e. my belief that Apostles are equal and there does not exist a universal primacy based on successors of only one Apostle. The canonical books in the Bible does not explicity describe such a supremacy, but there are at least two interpretations of certain verses in the Bible causing confusion (affecting the church in which I am member).
Any help appreciated.