The England international slotted home his fourth goal of the season after the hapless Stefan Savic had fouled Daniel Agger in the 13th minute.
City, missing key trio Vincent Kompany, David Silva and Yaya Toure, suffered back-to-back home defeats for the first time since February 2008 having lost to Manchester United in the FA Cup at the weekend.
It was the visitors who started the more composed and they should have taken a fifth-minute lead. Stewart Downing, playing on the left, played a short ball through the middle which Andy Carroll beat Savic to far too easily but the striker shot too close to Joe Hart with his weaker right foot and the goalkeeper made a good save.
Gerrard's corner was brilliantly volleyed by Downing, taking a deflection on its way through a crowd of players, and the goalkeeper again came to his side's rescue. However, he was powerless to stop Liverpool's next shot as from the corner Savic's foul on Agger - who fell theatrically - was deemed worthy of a penalty and Gerrard slotted home.
City started to get into the game as half-time drew near and substitute Samir Nasri drew Jose Reina into a save low to his left but James Milner blazed over City's best chance after Micah Richards had out-muscled Glen Johnson to get to the byline.
City started the second half better as Liverpool looked to sit on their lead and break on the counter attack and it required a timely intervention from Martin Skrtel to deny Sergio Aguero a shooting opportunity as he raced on to Adam Johnson's through-ball.
The possession was almost entirely City's - although Jordan Henderson should have picked out either Craig Bellamy or Downing after a long run - and Reina's save at the second attempt from Richards' close-range header maintained his side's lead.
Kolarov wasted a good opening by opting to shoot from distance with the defence back-pedalling before Reina did enough to put off Gareth Barry on the edge of the six-yard area.
With 10 minutes remaining Reds boss Kenny Dalglish sent on a sixth defender in Jamie Carragher to play a midfield holding role. Sergio Aguero flicked over a header from Nasri's cross late on but Dalglish's defensive ploy paid off as his side edged closer to a first final in six years.