MPU9250 Gyro Calibration-Offset
Temperature drift of the offset is +/-30°/s over -40~+85°C so it doesn't take much temperature delta to get 1-2.5°/600s.
If you know that it's not rotating you can correct the bias. If you have really deep pockets and probably appropriate security clearances you can get a much better gyro such as a ring laser gyro.
Or use GPNSS data fusion (using GPS, GLONASS, BeiDou, Galileo) to correct longer-term drifts and use the MEMS gyro for higher frequency information. In some cases you might be able to use a camera or magnetometer.