Gerrard, who scored the only goal from the penalty spot in the first half after Stefan Savic's foul on Daniel Agger, rejected the City manager's complaint over a tackle by Johnson on Joleon Lescott.
Mancini, whose side were without captain Vincent Kompany who was banned after being sent off for a two-footed challenge on Nani in the FA Cup defeat to Manchester United at the weekend, felt Johnson should have suffered similar punishment.
In the aftermath of that match, Mancini claimed that United's Wayne Rooney had tried to influence the Kompany sending-off.
Gerrard told BBC1: "It surprises me because he had a go at Wayne Rooney. Now he has had a go at one of ours."
But Gerrard warned his team-mates not to take a place in the final for granted.
He said: "There is no way this game is over, the job is half done."
Mancini admitted referee Lee Mason had been right to award the penalty but insisted he should have punished Johnson for sliding in two-footed on Lescott.
"I want to see the tackle by Johnson. I thought that was worse," he told BBC1.
Mancini admitted the absence of Kompany through suspension, David Silva with an ankle injury and with Yaya and Kolo Toure at the African Nations Cup, had made it hard for his team.
He said: "It is clear that without three or four players it's difficult.
"The result is not right. For me the result was a draw maybe."