He's a character in the *Percy Jackson and the Olympians* series, but his role is primarily in the second book, *The Sea of Monsters*.
In *The Sea of Monsters*, Gabe is revealed to be Luke Castellan's stepfather and a servant of Kronos. He's portrayed as a cruel and abusive figure who mistreats Luke and his mother.
While he doesn't play a direct role in *The Lightning Thief*, his presence is hinted at through Luke's troubled past and his resentment towards the gods, which is partly fueled by his experiences with Gabe.