He doubled down on that assessment in an interview with the Wall Street Journal, saying "I have thought about this for a long time and I am very, very worried." He also called the Ryan plan "politically catastrophic."
While there has been some reticence and hedging on the part of leading and establishment Republicans over the Ryan plan, this was blunt criticism from someone considered an "ideas man." Perhaps Gingrich is worried of losing his unofficial "GOP intellectual" title to the rising Ryan.
Going after Ryan was likely a purely political ploy by Gingrich, especially considering his earlier support for the Path to Prosperity. While Ryan's plans are bold and address a crisis that is paramount to the future of the country, they aren't as popular with the broader country as they are with the GOP base. Medicare reform has long been a toxic issue for politicians due to the power of the Seniors' lobby, and Gingrich is polling strongly in the Senior-heavy key primary state of Florida.
Newt would, of course, have to make it to Florida. We saw a strong early-runner in Rudy Giuliani last cycle bank on Florida only to be out of the race by the time that state's primary rolled around. And Allahpundit noted that he's in danger of being heckled out by the traditional GOP base.