Kevin-Prince Boateng was targeted by a small section of fans and, 26 minutes into the game, reacted by kicking the ball at them in anger before walking off the playing field. His teammates followed him.
"I've lived five kilometres away from Busto Arsizio for years and that area is full of foreigners," Gattuso told the Corriere della Sera.
"There are no racists in Busto. What happened yesterday was the fault of a group of imbeciles."
The Sion veteran then added: "How many times have there been boos against white players in the past? It's happened to me to, but I didn't give it any importance.
"Boateng certainly felt he was offended, but I continue to not see it as racism. It just seems to be the latest episode of collective idiocy from a minority.
"But if we are convinced that there is a racism problem then strong decisions need to be made. And yesterday's stance was strong.
"I, however, continue to believe that Italy is not a racist country."