The gyros are only used for coarse pointing, guide stars are used for much more accurate and precise position sensing during exposures (which is why they are called Fine Guidance Sensors).
gyros can be used for both sensing motion as well as creating motion (is this true for Hubble's gyros ?), using spinning spheres on each axis of a high enough mass ratio in comparison to the enclosing object similar to haptic ... if so, I wonder if the issue is a degraded gyro is impacting ability to steer the telescope not just to do the sensing
There's no good reason to combine jobs for such devices on spacecraft since the requirements for attitude control and attitude sensing are very different. The gyros on the HST are only for sensing, it uses a combination of reaction wheels and magnetic torque rods for attitude control, which are well suited to orbital observatory application due to their high precision.
