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!