This is actually an interesting topic. If you look at competitive states where Gingrich actually won a decent number of counties (MS, AL, FL, etc.), it looks like he did his best in White, non-Appalachian rural areas.
In the South, Romney tended to dominate with urban Republicans and those living in majority-Black areas (MS Delta, Black Belt, etc.) while Santorum was more preferred in (the very White) highland areas. Gingrich's support laid between these two extremes. I'm not exactly sure what that suggests about each of these candidates or their campaigns, but there does seem to be a gradient of Romney-to-Santorum support in the Southern contests that is directly correlated with Black %.
My theory is that racists and nativists were Gingrich-Trump voters, die-hard evangelicals were Santorum-Cruz voters, and anti-trade voters were Santorum-Trump voters.
