Struggle to breathe (4)
I believe the answer is:
'struggle to breathe' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for gasp that I've seen before include "Sudden intake of breath" , "Say "ah"" , "Draw breath sharply" , "Catch one's breath in astonishment" , "Pant" .)