Open-mouthed stare (4)
I believe the answer is:
'open-mouthed stare' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gape that I've seen before include "Look amazed" , "Look with mouth wide open" , "Yawn" , "Peer" , "Open-mouthed stare" .)