GULP:
to swallow rapidly, esp in large mouthfuls: to gulp down food
to stifle or choke: to gulp back sobs
to swallow air convulsively, as while drinking, because of nervousness, surprise, etc
"Gulp" can mean two things, same with the following of "Double gulp".
Meaning 1: Gulp can mean "Uh-oh!" or that you are scared. When someone replies with "Double gulp!" it means they agree that they are scared but they are more scared.
Meaning 2: It means the same as above, but to put it shorter, it just means that you agree with what the original "Gulp" was referring to.
