A gander is a male goose, and also an insult meaning "simpleton," a bit like calling someone "a silly goose."
Besides being the proper name for a male goose and a slang word for silly man, the word gander also shows up in the idiom "take a gander." The slang sense of gander comes from the meaning recorded in 1886, to take a long look by craning one's neck like a goose, or wander foolishly (again, like a goose).
