In which month of Elizabeth's pregnancy did Gabriel visit Mary?

Answer: The sixth

Gabriel came to Mary "in the sixth month" and told her that Elizabeth, once called barren, was in her sixth month.

“In the sixth month, God sent the angel Gabriel to a town in Galilee called Nazareth, Look, even Elizabeth your relative has conceived a son in her old age, and she who was called barren is in her sixth month.”— Luke 1:26, 1:36 (BSB)