The phrase "gaze open-mouthed" implies a sense of awe or surprise, so antonyms might include words like "disinterest," "indifference," or "nonchalance." For example, you could say that someone glanced quickly and disinterestedly at the spectacle, or that they looked away with an air of nonchalance. Other antonyms might include words like "ignorance," "unawareness," or "inattention," which imply a lack of curiosity or engagement. Conversely, some synonyms for "gaze open-mouthed" might include phrases like "stare in wonder," "gape in amazement," or "gawk in astonishment." These synonyms all emphasize a strong emotional reaction to the object being observed.