It is a verb:
to circulate salty water (or any other lotion or liquid) in your mouth when you have got cold, in order to make your throat smooth and make the itchy throat cooler.
Gargling takes place at the point where your mouth meets your throat, and with your mouth open. I would say that you slosh or sloosh liquid around your closed mouth, coating your cheeks, gums and tongue with it.
I agree, however, that gargling is more likely to be done when you have a sore throat.
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