Jayne's punning 'Guy folks' is dead accurate. 'Guy' does come from Guy Fawkes. He was, is, burned in effigy on the 5th of November and children made their own guys, dressing these in old clothes. So the word got transferred to any scruffy man and thence became a friendly word for a man, then for any man. That usage crossed with settlers in America but the word died out here, just as 'gotten' and 'faucet', for any tap, has.
The influence of American films and TV caused its revival here. And why is it now used for both sexes ? A consciousness of sexual equality, though I think it sounds odd when used for women. I don't think it is used in the singular for a woman, but was used in the plural for a mixed group of both sexes and that use has been extended to a group of women ; "Hey, you guys!" is now addressed to both single sex and mixed groups.