Gaius Flavius Aurelius Gallienus (78 BCE - CE 55) was born Gaius Flavius Theodorus as the eldest son of the elder Gaius Flavius Theodorus Aurelianus. After Aurelianus' death in a slave's rebellion, the young Theodorus was adopted by his paternal uncle, Marcus Aurelius Celsus and became Gaius Aurelius Celsus Flavianus as his heir. Later, Theodorus, then referred as Celsus, also became the elder Aurelius' son-in-law after being married to the latter's daughter and his own cousin, Aurelia Theodora. The elder Aurelius died in a stroke, leaving his will that desired the firstborn son of Celsus to be an Aurelius, which hinted his permission for the younger Aurelius in case the latter would want to become a Flavius again. Celsus then renamed himself as Gaius Flavius Aurelius Theodorus, remained not an Aurelianus but an Aurelius to honor his adoptive father's glory.
Two years after the death of Marcus Aurelius Celsus, Aurelia bore Theodorus two sons, the elder named Marcus Aurelius Celsus after their own maternal grandfather and the younger named Gaius Flavius Theodorus after their father.
There might have been some time in his life, Theodorus was referred as Theodorus Flavius Aurelius and (Gaius Flavius) Theodorus Pius.