Gaggling is an oral sex act involving any phallic object, though typically a penis, in which the a person (the gaggler) recieves the phallic object in to their mouth and down the back of their throat (deep throat) while keeping their mouth wide open.
Gaggling is often accompanied by a gag reflex which produces an excess of saliva. The gag reflex must be controlled in order for the gaggler to gaggle on the phallic object.
It is not uncommon for excessive amounts of saliva to run down the gagglers chin, as saliva produced by the gag relfex and cannot be orally contained due to the gagglers open mouth.
It's the feeling you get when someone says something so gross and yet, so hysterical, you don't know whether to laugh or gag. Or both. Therefore the feeling of gaghling.
SHANNON: My friend found many mice in her room, they caught some with mousetraps, and they took a picture of their guy friend with a face of disgust and an open mouth, and it looks like he's going to eat them, little mice appetizers. Ana: OMG, I don't know whether to laugh or gag right now, I'm gaghling! EW!