The former Italy international still regrets his actions but explained that everything was sorted out via SMS and that the pair now get on just fine
Leonardo angered AC Milan fans by taking charge of great rivals Inter in December 2010, just over six months after standing down as Rossoneri coach.
The following May, during Milan's title celebrations, Gattuso was caught on camera singing along to a song berating the former Selecao star over his perceived act of betrayal.
"When I went with the ultras to sing, 'Leonardo, man of sh*t', it was not a nice thing," the World Cup winner admitted to France Football. "It was a mistake on my part.
"But I apologised to him for that and, since then, things have been clarified by SMS.
"One may have had problems with someone, but you should not hold onto the bitterness. I made a mistake and I apologise."
Gattuso never had any such problems with PSG's current coach, Carlo Ancelotti, who led Milan to two Champions Leagues and one Scudetto during his eight years at San Siro.
"This is someone you would always have as a friend," the combative midfielder enthused. "He is friendly, he never wears a mask ... Carlo is Carlo: a great."
Gattuso, who was capped 73 times by his country, parted company with Milan at the end of last season before subsequently signing for Swiss side Sion.