Calibrating the rate might not be the issue. Gyros always give you delta rate. Even at rest they will output a delta rate which is the bias and random angle walk. In order to get the absolute heading you have to integrate the delta rate and over time you get some very serious errors. Rate gyros are very good when you have some rapid rotations, but they will not be able to keep an accurate direction for very long.
If however you want to calibrate the rate output, you need nothing more but a Parker rotary stage. This will set you back a few grand. If you need to remove the bias and random angle walk (which is the real problem). The gyro must be placed exactly "flat" onto the rotary platform or the results will be slightly wrong
But there is an even easier way to calibrate a rate gyro (poor man's gyro calibration). Mount the gyro into a cubic frame... exact. But it on the table, start you integration and rotate the frame 360 degrees in a way that the gyro detects the rotation. stop integration. The results are surprisingly accurate. If you do it quickly the bias will not impact your result very much