Middle English gandre, gander, going back to Old English gandra, ganra, going back to Germanic *gan-ra- (whence Middle Low German ganre "male goose," Upper German dialect Gander, Ganter), derivative from the base of Germanic *gan-s- "goose" — more at goose entry 1
Noun (2)
probably derivative of gander entry 1, from the goose-like appearance of a person stretching to look at something