Olivier Ntcham scored the only goal of the game, however the former Liverpool captain believes that the goal should not have stood.
Gerrard told 'Sky Sports News' that he thought there was a foul in the build-up to the goal.
Gerrard said: "It's definitely a foul. I've been around the game a very long time, and that's a foul in my book. It's actually a foul in the fourth official's book as I can hear him going down the mic, 'Foul, Foul, Foul'.
"The referee's ignored the information and they go and score a goal. I just went in and had a calm chat. I wondered why he hasn't seen a clear swipe from Rogic on Jack. It's blatant. For me it's 100 per cent a foul.
"If we'd lost the game with a moment of class or brilliance from Celtic I'd have held my hands up, but the referee's cost us for sure."
Celtic's game plan was to defend and catch the hosts on the counter-attack.
September 2, 2018
"I thought we played better in the second half," Gerrard added.
"We kept the ball better and we made passes and created a few decent openings. With a little more quality we could have got a goal.
"I've asked questions of this squad since the June 15 and we've closed the gap. But there's still work to do. We still need to improve and get bodies back that are fit. We need to focus not on officials, if we can help it, but on ourselves.
"Hopefully, in the near future we'll get more training time and when Celtic next come around, we'll turn up for 90 minutes and not just the 45 today in the second half."