I don't trust my firefox thing, wasn't isn't a real word in its book. Same with hasn't and I forgot the rest.
Wasn't and haven't totally aren't real words. They're contractions, and thus mean they're typically two words shoved together in a semi-holy manner. The spelling is "gist", as when people say "jist" I think they're trying to use the word "jest", which means something entirely different.
most often, they probably don't know how to spell it. that's the beauty of spoken word, you really don't need to know how to spell anything.
i've alwasy seen the essence of context spelled with a "g". jist looks like a misspelling of jest. the gist of jest is supposedly the pratfall.