I agree with Flah and aaa here. Gingrich is pandering. He might not be the sharpest knife in the drawer, but I don't think he is this bloody stupid. This is pandering to the far right. He wants to run for president and he knows that he doesn't stand a chance in the Republican primaries if he can't get the far right nutters to support him. He's trying to steal votes that might go to the likes of Huckabee or Palin (should they choose run, God forbid).
I also think that Tolpuddle Martyr brought up an interesting point. Not all Muslims are Arabs and not all Arabs are Muslims. However, the stereotypical Muslim is an Arab and vice versa. Because being blatantly racist and saying something like "Arabs" or "brown people" is universally frowned upon, I don't doubt that certain people use the word "Muslim" as a sort of code word for "Arab" or "person with darker skin than me." So if Gingrich really means he's afraid of America becoming a country full of atheists with dark skin, that really wouldn't surprise me, and his fear at least wouldn't be contradictory.
And I really hate people who try to use Muslim as a code word for Arab. Not only for being bigots, which is reason enough, but also because they make it even harder for sane people to make legitimate criticisms of Islam without being considered a racist.