Gattuso's men meet Ronaldo and Juve in the Supercoppa Italiana in Saudi Arabia on Wednesday.
Gattuso said the Portuguese star had changed in recent years after initially being too focused on "doing a lot of show".
"I think Cristiano has become an incredible player over the last seven or eight years," he told a news conference.
"When I met him a few years ago he was doing a lot of show but now he stopped doing it a while ago. Now, he does everything with vehemence, attacking with depth.
"He wants to score and wants to give you hard times and put the ball into the net.
"He has changed in the last seven or eight years, now he is the perfect machine. The older he gets the more he sprints.
"He seems a bloke without age, here you understand what kind of top player he has been over the last eight years."
Juventus and Milan are bidding to win a record eighth Supercoppa crown when they meet in Jeddah.