"Guy" wasn't a word until the early 1600s when Guy Fawkes attempted to blow up the English Parliament in the Gunpowder Plot. The name became an insult, and addressing a group as "you guys" would have been met with hostility.
Guy was an uncommon name in England, but may have been popular in York on account of a local notable, Sir Guy Fairfax of Steeton … Immediately before his execution on 31 January, Fawkes fell from the scaffold where he was to be hanged and broke his neck, thus avoiding the agony of being hanged, drawn and quartered. He became synonymous with the Gunpowder Plot, the failure of which has been commemorated in the UK as Guy Fawkes Night since 5 November 1605, when his effigy is traditionally burned on a bonfire, commonly accompanied by fireworks.
Over the next several hundred years, though, "guy" became viewed more positively, becoming a word for every man. Starting in the mid-twentieth century, it sometimes refers to women, though such usage is problematic given its masculine history. As a given name, its popularity has waned in recent decades, in roughly inverse proportion to the word's use as a plural noun.