This will set the gyro to zero without recalibrating it. The effect is instantaneous.
SensorValue[gyro] = 0;
Note that a big source of drift with this type of gyro is vibration. The gyro should be mounted on VEX rubber links or suspended in a rubber band sling.
@jpearman tested a couple of different drift solutions and determined isolating the gyro from vibration is very helpful.
Here’s the thread: