Steam Input can auto-calibrate the gyro during regular use, just head over to Steam Big Picture Mode > Menu [Press the Guide button] > Settings > Controller > Calibration & Advanced Settings > Gyro Calibration and press the Calibrate button and follow the instructions. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.