Another opinion:
A Browser's Dictionary, by John Ciardi, published as A Common Reader Edition by The Akadine Press, 1980, doesn't approve of the allusion of the expression "Great Scott" to Winfield Scott. A Common Reader Edition indicates that "Great Scott" is derived from the German expression "Gruess Gott!" and that it has been an Americanism only since the 19th Century. This suggests a borrowing from the greetings exchanged by German Immigrants, their cordiality contributing to the exclamatiory sense of the American adaptation. [Capital Scot advisory: Several on-line searches trying various arrangements of the terms "Gruess Gott" and "Great Scott" yielded no results. Perhaps the print media are still ahead of us on this one?]