Rudder Common Gain is basically identical to Rudder/Gyro Gain in your radio. It is only making the range wider if combination with 100% in your radio and 1.0x is not enough.
So to allow even higher gain there is such multiplier. You can set it to 1.5x for any model, but your Gyro Gain will be just low on some.

The best to try is remove all blades, disconnect tail push rod from your servo, spoolup the motor to your flight headspeed and then carefully move with the push rod by hand. Check how smooth it is moving. This condition can be very different from what you are seeing in the flight.

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Spirit System developer