Gavin's father was the king of a planet who had travelled to Earth for in 1997. He mated with a human woman, producing Gavin. Unwilling to have his people come looking for his son, he gave the woman a bio damper for the boy before returning home. It prevented Gavin from being found by masking his alien biology. After Gavin's mother died, he was sent to live with his aunt and uncle, who cared little for him. He was interested in animals and poetry.
Because he was half-alien, Gavin was left on the Earth when, in 2010, the Automatons of his homeworld, searching for him after his father's death, used Warp shunt technology to remove all humans and Mr Smith from Earth except for Clyde and Rani, who had been grounded by the Judoon.
Clyde and Rani helped Gavin discover who he really was. He ordered the return of the humans and proclaimed Rani and Clyde lords. After she returned to Earth, Sarah Jane had Mr Smith edit records to show that Gavin had gone to live with relatives in Australia. (TV: The Empty Planet)