One of the first revelations is the discovery of James Prat, Gary's great-great-great grandfather.
He was born in 1818 in the village of Hinckley — not far from Leicester, where Gary was brought up — and the TV star soon finds out that James served six months in Leicester Prison for stealing six chickens in 1848.
"Four of the fowls were in the pot and two were up the chimney," one reporter wrote of James' arrest.
Along with chicken rustling, James was also charged with night poaching and malicious trespass and served repeated terms in the jail, including spells in solitary.
"Some of the poachers' tales of how he got caught made me laugh. But it's actually a terribly sad story," said Gary, after discovering that his ancestor had to poach birds to feed his eight children.
Another intriguing tale emerges from the family tree. Hidden among his blue-collar ancestors, Gary finds Tomas Billingham, his four-times great-grandfather who was born to an illiterate gardener but went on to become a middle-class law writer.
But how did he end up becoming a highly skilled legal clerk from such humble beginnings? Gary's investigation takes him from Tomas's old school to the site of his workplace in Tottenham.
"I've become emotionally attached to them," Gary says of his spirited forebears. "Different lives, different issues, big problems, but good moments. I'm proud of them both, I think."
Tune in to see Gary on the trail of his ancestors on Who Do You Think You Are? Wednesday 21 August, 9pm, BBC1.