I was just using commercial products to point out that you can't expect a $30 toy to be perfectly accurate.
The HT gyro is completely analog. The bias is susceptible to power supply voltage changes (like when you run a motor). Being analog, it is limited to the 10-bit ADC of the AtMega48 co-processor in the NXT, and also to the update rate of 333Hz (not sure that an NXT integration program would be able to keep up with that anyhow). Each change in the raw ADC value represents 1 DPS of rotation (low precision). All of my HT Gyros have a different bias.
The dIMU gyro is digital. It has 1/16th, 1/64th, or 1/128th DPS precision (depending on the range mode). See my report on the dIMU
that I wrote a couple years ago, around the time of the dIMU release. As far as I can remember bias is extremely minimal, so calibration/offset hasn't been necessary, at least for my applications (primarily segway type robots).
I don't have any mindsensors gyros.
If you want to try to get a more accurate integration with the EV3 Gyro, you could read the rate value and integrate it in your EV3 program.