Given the name, it's likely that what this device actually does is reduce the effect of natural gravity and inertia on the ship and its crew, so to be accurate, the ship could still turn, but doing so would probably kill or damage everyone and everything on board. The fact that Wash was unable to steer was probably a failsafe that disabled maneuvering if the system detects that the Grav Dampener is offline.
This technology would appear to be analogous to the Star Trek technology ‘Inertial Dampeners’.
It has been left in the generic 'Technology' category as well because it (or something very like it) is probably used in a number of terraforming operations.
Most internal combustion engines also sit on a device like this to keep from shaking itself (and possibly the car as well) apart. usually a urethane bushing with a bolt between it and the motor mount sitting on the motor.