If he said Kyle Walker is an irreplaceable player this means he won't allow Walker to leave Manchester City for Bayern Munich or any other team even if they sell this player finding another good player to fill his place can't be easy.
Kyle Walker is one of Manchester City's best defenders, so if Manchester City leaves him at the moment, it will be very difficult for Manchester City to find a defender like him. Kyle Walker wants to play for Manchester City if Manchester City keep him in the regular XI. I have learned from several media that Guardiwala will not consider him in the regular XI if Manchester City keep him at the club. I think his performance is still needed by Manchester so he can still be included in the regular XI. Although he missed the end of the last season due to injury, he kept the team's defense strong from the start.