The defensive midfielder was widely expected to leave the club during the close season transfer window and has not even sat on the bench so far this campaign.
His return for Sunday's league fixture at Udinese came as Italian media reported that midfielder Lucas Biglia could be out for two months with a calf muscle injury.
"It's all been pub talk about him (Montolivo)," Gattuso told reporters on Saturday. "He's never been out of the squad, he's always been training with us. I make the choices but there has never been a problem."
"I don't close the door for anyone if they train the way I tell them to," he added.
Montolivo is in his seventh season at Milan since joining from Fiorentina, although he has been plagued by injuries.
A regular for Italy at one point, he missed the 2014 World Cup after suffering a broken leg in a friendly against Ireland shortly before the tournament. He also fell victim to a knee injury which made him miss most of the 2016-17 campaign.
He fell out of favour in the second half of last season, after Gattuso was appointed coach and was sent off on his last appearance, against Atalanta in May, after coming on as a substitute at halftime.
Milan climbed to fourth with a 2-1 win over Genoa on Wednesday, putting them in the Champions League places.
"We did so much to get into the Champions League zone and now we need to stay there," said Gattuso, who added he was unhappy with Saturday's training session.
"Tomorrow, I want to see more determination, more hunger and more cunning," he said.
(Writing by Brian Homewood; Editing by Christian Radnedge)
