The Portugal star hit a 50th career hat-trick to take his league tally to 22 goals for the season in another scintillating performance at the Bernabeu.
Girona, who beat Real earlier this season, caused them plenty of problems again with Cristhian Stuani scoring twice and Juanpe also on target.
But there was no stopping Ronaldo, who has now hit an incredible 17 goals in his last eight appearances in all competitions.
“Ronaldo is very important to us, he transmits an energy across the whole group,” Zidane said in Marca.
“He knows himself very well and he knows when he has to stop a bit so that, when the end of the season arrives, he hits top form.
“He is tremendous. In training he is incredible. For example, if there is a penalty, he takes it with maximum concentration. And that is only training. It is incredible.
“But I also want to highlight his colleagues, who help him a lot. We scored some spectacular goals that came from the whole team.”
Lucas Vasquez and substitute Gareth Bale also found the net for Real, who looked sensational going forward but remain vulnerable in defence.
“We have regained confidence and that has been evident in our home games,” added Zidane.
“We have scored many goals in four or five games at the Bernabeu, which we didn’t do at the start of the season. And then there is Cristiano. When Cristiano is fine, the team is fine. And then it’s important that the team doesn’t take its foot off the pedal.
“We have to improve things (defensively). I’m not happy with the three goals conceded and that has to be improved, we have to work on it.”