I think that Gingrich is taking a real chance by saying this sort of thing. He is likely to anger the type of people who are most likely to vote in Republican primaries. If he really wants to be the Republican nominee for the presidency, this is a risk.
Many people are going to (and already have) been calling Gingrich's proposal a type of "amnesty." Republican voters hate the idea of letting anyone who came into the country illegally have a path to citizenship, no matter how difficult it is. Gingrich is not likely to pick up many moderate votes given that he would require people to have been in the US 25 years to qualify.
So Gingrich is risking angering conservatives without pulling in moderate votes.