Gerrard spoke to the press after the deal for Manchester City's Kean Bryan fell through, and said: "We need a centre-back."
19-year-old Kyle Bradley was given 45 minutes in central defence during Rangers' 3-0 friendly win over Wigan.
"We have loads of targets that we have been monitoring," Gerrard, who has already signed 10 players, said.
"Unfortunately, we couldn't get an agreement with Kean. He wants to keep his options open and we respect that."
"You can see quite clearly that we need a centre-back so that is definitely an area that we are trying to add in. Hopefully it will be sooner rather than later."
Gerrard has already added to his defence with the signing of Connor Goldson, Jon Flanagan and Nikola Katic.
However, the new Rangers boss has lost David Bates, Bruno Alves, and Russell Martin.
Hearts have rejected a £200,000 bid for Kyle Lafferty, a former Rangers striker.
On Thursday, Rangers will host Croatian side Osijek, with a 1-0 lead from the first leg in the bag.
Gerrard would like to strengthen his squad before they travel to Aberdeen.
"I would love that," he said. "I hope it is tomorrow. I am in a rush."