He said he expects Mitt Romney will be the nominee and called on the party to unite behind the former Massachusetts governor.
"You have to at some point be honest about what's happening in the real world as opposed to what you would like to have happened," Gingrich told supporters at a suburban Charlotte, N.C. restaurant the morning after Romney swept primary contests in five states.
"Gov. Romney had a very good day yesterday. You have to give him some credit. He's worked for six years. He put together a big machine ... I think I would obviously be a better candidate."
But Gingrich said GOP voters didn't agree, and that he would begin working to unite the party.
"I also think that it's very, very important that we be unified," he said. "No conservative anyplace in America should have any doubt about the importance of defeating Barack Obama."
Gingrich did not formally withdraw from the race but said he is now campaigning as a "citizen." He did not explain what he meant.
"I've been coming here a long time as a citizen. I will keep coming here as a citizen. I have a schedule for the rest of the week as a citizen," he said, adding that he will talk about economic issues, such as high gas prices and high unemployment, that could help the GOP capture the White House and the U.S. Senate and grow the Republican majority in the U.S. House.
"We are going to stay very, very active and we are working out the details of our transition," he said. "But I am committed to this party. I am committed to defeating Obama. We will find ways to try to be helpful. But I think it's pretty clear that Gov. Romney is ultimately going to be the nominee."
Gingrich spent Tuesday campaigning in the Charlotte area. He planned to go ahead with several scheduled stops in North Carolina in the coming days.