When you gargle, you expelled air in a very similar way to the one that you need to produce the French 'r' sound. Awareness is key on this trick, you need to realize and understand the way you’re expelling air.
The next step is to reproduce the gargle with no water. Now just try to modulate the air and naturally the ‘r’ sound will be produced.
Congrats! Now you can sound more natural in French.!!
