Caspar René Gregory published a cataloging system for the Greek New Testament manuscripts in 1908. In 1963, Kurt Aland expanded Gregory's work, and the Gregory-Aland (GA) numbers became the industry standard for referencing Greek New Testament manuscripts.
APPENDIX 2 Bible Texts and Versions –Why We Need to Know
We must face the reality that while the original 39 OT manuscripts and 27 NT manuscripts were inspired by God [Lit. “God-breathed”] (1 Tim. 3:16), as the authors were moved along by the Holy Spirit (1 Peter 1:21), this was not the case with the copyists thereafter. Yes, hundreds of thousands of scribal errors crept into our manuscripts.