Heap as an adverb for "very", "a lot", or "very much" is a pidgin English word that used to be common in depictions of the speech of American Indians (native Americans). Here are a few quotations from the OED:
1867: "Disturb the game and you make the Indian 'heap big mad'."
1872: "'Heap' is 'Injun-English' for 'very much'."
1902: "Billy explained..'she heap much hungry'."
1958: "President Coolidge posed later in the regalia of a heap-big chief."
I'm not sure how much real native Americans used the word "heap" in that sense, but it was certainly common when white people were pretending to talk like native Americans. That is, it's intentionally incorrect, nonstandard English: an easy way to "sound like an Indian".
Of course, once that usage is established, people can use it playfully or informally for all sorts of reasons, which may explain its appearance in that line in Gone with the Wind. The last quotation above is by humorist Bennett Cerf, using the word playfully to say that President Coolidge was dressed as an Indian chief.