E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A contraction of God be with you. Similar to the French adieu, which is à Dieu (I commend you to God).
Some object to the substitution of God in this phrase, reminding us of our common phrases good day, good night, good morning, good evening;
Good be with ye would mean may you fare well, or good abide [with you]