gillion vs zillion

gillion

noun

  • An unspecified large number (of). 

num

  • A thousand million, proposed as an alternative to the (now little used) British milliard and the (ambiguous, in the UK) US billion. 

zillion

noun

  • An unspecified large number (of); a gazillion. 

How often have the words gillion and zillion occurred in a corpus of books? (source: Google Ngram Viewer )