Gargle is more known as ring modulator and attenuates the sound within specified time period. It's useful for tremolo and "low bit depth" robotic effects.
First of all, make sure that DryMix is checked, it will sound bad otherwise.
Rate: Rate in hz. Use 4-8hz for tremolo (lower for guitars, xylophones, vibraphones and similar, higher for strings and long synths) and 30+ for robotic effects (60hz is good).
Wave shape: 0 is sine and 1 is square volume envelope. Square sounds more agressive but the clicks it produces might be unacceptable in some cases.