The cooking star told The Sun that he "felt ashamed" about the attack by his former babysitter's husband and accepted that he "should have told somebody" before now.
Recalling the incident in an extract from Life On A Plate published by the paper, Wallace wrote: "George loved watching us play our soldier games and we spent hours talking.
"One rainy Monday afternoon... his wife was out and George said, 'Gregg, come here and I'll teach you what happens when you get a girlfriend'.
"Before I had a chance to ask what he was on about his hot stinking tobacco breath was in my face, his lips were stuck on mine and he was poking his tongue into my mouth.
"'Well done, Gregg', he said. 'Now I need to show you what the girlfriend does to you'... He grabbed my right hand tightly and, in one swift movement, shoved it down his pants."
Wallace went on to reveal that George warned him not "to cause trouble" by telling anyone, and later accused the youngster of having "let [him] down" by refusing to join him for a "lie down" in the bedroom.
Wallace stated on Twitter after the piece was published that it "took a lot of courage to write".
Life on a Plate is released on October 25.