During the The Secrets of Vesuvius, Gaius owned a beautiful villa located near the foot of the volcano Vesuvius; it is here that he is first introduced to Miriam bat Mordecai, who has come to stay along with the four detectives. The two quickly fall in love and a few books later they are married. Gaius' villa, however, was lost in the volcano's eruption.
In The Slave-girl from Jerusalem, Gaius and Miriam find out that they are to be parents. Unbeknownst to Gaius, Miriam has a dream in which she is told that she will have twins. She realises that she will not survive the birth if she wants her babies to live, however she decides to tell only her closest friend, Hephzibah. Miriam dies in childbirth, but her twin boys, Popo and Soto survive.
After the loss of his wife, Gaius falls into a state of extreme grief and no longer seems to care whether he lives or dies. He disappears and for several months, both Flavia and Jonathan's families think he drowned in a shipwreck. The four detectives later find him in The Beggar of Volubilis, drunk and gambling in Roman Africa. Flavia at first struggles to persuade him to return home with them, but he eventually relents.
In The Prophet from Ephesus, Gaius' son, Popo, appears to have been kidnapped along with several other children. The four friends search for him, going halfway across the world, but it is in vain. The only information they discover is that he was not kidnapped with other children, but by his wet-nurse, Lydia. Popo has still not been found by the end of the series.
Gaius is first introduced as an amiable, hardworking man. However, after the death of his wife, he becomes reckless, depressed and immoral.
Gaius looks strikingly like his brother, apart from his nose, crooked from being broken once. He is tanned from working outdoors.