The Premier League leaders scored three times after Gravenberch received a second yellow card in the 62nd minute. Erling Haaland opened the scoring in the 65th minute, Jérémy Doku doubled City's advantage three minutes later and Rayan Cherki completed the victory in the 78th minute.
Iraola believed Liverpool had been the better side before the dismissal.
“I think it's obvious. We've played two games today. One game until the 60th minute and then another game after the red card. We were the best team in the first game and then they were better,” he told Sky Sports.
The Liverpool manager questioned the decision to book Gravenberch for his first offence, insisting the midfielder had not committed a yellow-card challenge.
“The first one is not a yellow. Anderson carries on a couple of steps; he wasn't even going to give a foul. I've no idea why he gives a yellow card. We have like nine players behind the ball.”
Iraola said playing against City with ten men presented a different challenge, particularly after conceding so soon after the dismissal.
“It's already difficult to press them when it's 11 v 11, but with one less, it becomes a different game and then we concede straight away before making a substitution. You have to play differently when you're 1-0 down.”
Despite his frustration, Iraola praised Gravenberch's performance and the effort of his players, while also thanking Liverpool's supporters for their backing throughout the match.
Maresca acknowledged that City had struggled before half-time and said tactical adjustments helped his team take control after the break.
“Very difficult circumstances, difficult environment. We struggled first half, we were on the ball but not so much off the ball. Second half we made some adjustments and we were much better,” he said.
The City manager also addressed the hostile atmosphere and the controversy surrounding the club, saying his players had to concentrate on what they could control.
“I can not control things outside football so why stress? I stress about gameplan, day by day training sessions but the rest it's not in my hands.”
Maresca praised Doku's contribution but warned that the winger, recently returned from injury, needed careful management.
“Outstanding — Jeremy [Doku] was very good we need to take care of him. He is important but only just back from injury. Performance from everyone was brilliant.”
Manchester City's victory preserved their perfect league record. Iraola, meanwhile, said Liverpool would assess their injured players before travelling to Austria, admitting it would be difficult for them to return in time for the next fixture.