gaper

intransitive verb gaped, gaping

1. to open the mouth wide, as in yawning or hunger
2. to stare with the mouth open, as in wonder or surprise
3. to open or be opened wide, as a chasm

noun

1. the act of gaping; specif.,
   1. an open-mouthed stare
   2. a yawn
2. a wide gap or opening
3. Zool. the measure of the widest possible opening of a mouth or beak