Another Reason the Tyndale House (Cambridge) Greek New Testament is A Remarkable Resource

γεινομαι was the normal spelling in Luke. It’s not a misspelling, but a prestigious koine spelling used by careful scribes to bring out the long vowel which arose when the second gamma of the Classical form γιγνομαι was dropped. You can call it a ‘historic spelling’ if you like and claim it has nothing to do with pronunciation, but that just makes the scribes smarter that they were able to preserve into the fourth and fifth centuries spellings representing pronunciations which were no longer current.

Read the post for the evidence.