Bengal confirmed it's "Ghast", and it is not a mistake.
http://forums.myvmk.com/showthread....s-(just-for-laughs)/page2&p=114587#post114587
http://en.wikipedia.org/wiki/The_Empty_Child
It comes from THIS episode of Doctor Who, so I've always assumed its a more old and british spelling. Let's take a look at the meaning of the word "Ghast" to see if its being used in context:
It also says "Ghast" is the archaic form of "Ghastly"... Now in Doctor Who, (in the episode), it didn't protect against "Gas". The mask was quite just for scary "ghastly effects". Maybe Amy took this into account when naming it. Or maybe...it doesn't really matter at all. You guys decide.