Origin and history of gonzo

"a large waterfowl proverbially noted, I know not why, for foolishness" [Johnson], Old English gos "a goose," from Proto-Germanic *gans- "goose" (source also of Old Frisian gos, Old Norse gas, Old High German gans, German Gans "goose").

This is reconstructed to be from PIE *ghans- (source also of Sanskrit hamsah (masc.), hansi (fem.), "goose, swan;" Greek khēn; Latin anser; Polish gęś "goose;" Lithuanian žąsis "goose;" Old Irish geiss "swan"), probably imitative of its honking.

Geese are technically distinguished from swans and from ducks by the combination of feathered lores, reticulate tarsi, stout bill high at the base, and simple hind toe. [Century Dictionary]

Spanish ganso "goose" is from a Germanic source. Loss of "n" sound before "s" is normal in English (compare tooth). The plural geese is an example of i-mutation.

The meaning "simpleton, silly or foolish person" is from early 15c.; proverbial expressions indicating "know no more than a goose" are by late 14c.

The bird's reputation for stupidity is not classical. Roman geese were sacred to Juno and held in highest honor. In Athens (as also in Rome) they were noted for watchfulness, and as an erotic bird. Tales told of various geese enamored of a boy, a physician, a philosopher (see Thompson, "Glossary of Greek Birds"). It was a term of endearment in later Greek (compare duck (n.1) as an Elizabethan term of endearment).

To cook (one's) goose is attested by 1845, of unknown signification; attempts to connect it to Swedish history and Greek fables are unconvincing. Goose-egg "zero" is attested by 1866 in baseball slang, from being large and round.

The goose that lays golden eggs (15c.) is from Aesop. In Homer as in Middle English typically with the epithet "gray" (argos). Thompson notes that "The Geese in the Odyssey are tame birds, ... in the Iliad always wild."