* Surprise: "The little girl's mouth gaped open as the magician pulled a rabbit out of his hat."
* Horror: "As the monster emerged from the shadows, everyone's mouth gaped open in horror."
* Awe: "Standing before the magnificent waterfall, her mouth gaped open in awe."
* Speechlessness: "He listened to the news, his mouth gaped open, unable to speak."
* Yawn: "The professor's lecture was so boring, everyone's mouth gaped open in a yawn."
Remember that "mouth gaped open" is a strong phrase that conveys a strong emotion, so use it sparingly.