[Bongle]

This is called bias, and is usually present in any sensor like the gyro.

There's a couple fixes, depending on how much work you want to do
1) Ignore it (it'll still be accurate enough over short periods)
2) Try to calculate the bias when the robot is known to be sitting still (I believe the gyro code already does this, but you can add a longer bias calculation for a bit more accuracy)

I think you should be able to buy a gyro chip with better specifications that will drift less if it really is a deal-killer.

If both gyros have bias of the same direction and same magnitude (and have similar specifications), I always wondered if mounting them both on top of each other would allow the error to cancel out.