TV footage captured on the father of five, who hails from Barmouth, Wales, impersonating a monkey in the direction of the Old Trafford left back, who had been at the center of the racism controversy that saw Luis Suarez banned for eight matches in late 2011.
Images and footage of the former carpenter circulated on social networking website Twitter and complaints regarding his actions were made to the police.
Gannon claimed he was impersonating a Neanderthal and pleaded not guilty to a racially aggravated public order offense and using threatening or anti-social behavior causing harassment, alarm or distress.
However, after a two-day trial at Liverpool Magistrates Court, he was convicted of the offense and the 58-year-old will be sentenced this Friday afternoon.
“This gesture was caught on camera and it was seen globally not just nationally," prosector Rob Girvan said. “There was a volatile and tense atmosphere at the game because of a player on the pitch who said he was a victim of racial abuse.
“The defendant saying he was impersonating a Neanderthal is just a childish excuse to cover up what he did. It was clearly a racist gesture.”