What was Caesar Augustus' real name?

Answer 1

GAIUS IULIUS CAESAR

Note that the letter "J" only became a separate letter from "I" in the Middle Ages.

Also, the praenomen was rarely used to refer to people (that is the first name, Gaius.) His cognomen, Caesar would have been used in conversation.

Answer 2

Octavius - Gaius Octavius Thurinos (complete name)

He became the First Roman Emperor and Changed his name to Gaius Julius Caesar Augustus.