This expression means “take a look,” “get a peek,” “check it out,” and so on.
‘Gander’ means ‘male goose’ and derives from the Old English ‘gandra.’ The roots of the expression are also from England. A work of 1887, The Folk-Speech of South Cheshire, says, “Gonder, to stretch the neck like a gander, to stand at gaze.” The next known example is from the Cincinnati Enquirer in 1903: “Gander, to stretch or rubber your neck.”
Around the 17th century, ‘to gander’ also was used to mean ‘to wander foolishly or aimlessly.’ Which geese appear to do, but how can we know what they have in mind? They may be thinking very serious thoughts.
So how did ‘take a gander,’ essentially meaning ‘take a male goose,’ come to mean ‘take a look’? Probably because geese have long necks and like to poke their heads just about everywhere and twist their necks to stare at anything interesting. Geese are the archetypal rubberneckers.
No doubt ‘to gander’ became the term because ‘to goose’ had already been borrowed; this was taken from the way that the birds were known to put their beaks embarrassingly — and sometimes painfully — into one’s more private places.
Incidentally, the word ‘gaggle’ means a group of geese on the ground. When they are in the air, they are called a ‘skein’ when flying in a V, or a ‘plump’ when flying in a close-knit group.
As for me, I’m going to do what I do every Sunday morning: go to the grocery store and take a gander at what looks good in the produce section. I promise not to goose anybody!