Seems to me that when I was a teenager, getting into the '70s, 'Get outta here!' was a not-uncommon expression of scornful incredulity, and was soon shortened to 'Get out!' That was in southern Ontario. The usage of 'Get out!' to express often-joyful surprise is newer, in my experience.

In the Maritimes, it was 'Go 'way with you!' or 'G'way whicha!' or just 'Go 'way!' or 'Gway!'. This was clearly an older and more established idiom.