General descriptions:
* Gaping: This implies a wide-mouthed, surprised, or amazed stare.
* Staring: This is a neutral term, simply indicating looking fixedly at someone.
* Gazing: This suggests a more contemplative or admiring stare.
* Mouth agape: This is a more literal description of the open mouth.
More specific descriptions:
* Astounded: This implies being struck with amazement or wonder.
* Speechless: This indicates being so surprised or shocked that you can't speak.
* Awe-struck: This means being filled with a sense of wonder or reverence.
* Bewildered: This implies being confused or puzzled.
The best word to use will depend on the specific context and the emotion you want to convey.