Why does it mean to say a gyro has an even degree of freedom?
Why does this degree correspond to the number of gimbals ?
A gyroscope can move freely about its spin axis, this equates to a single plane of movement but is not defined as a degree of freedom. A gimbal allows movement around another plane in addition to the spin axis, this is called one degree of freedom. The number of degrees of freedom equals the number of gimbals.