Oscillatory attitude motion of gravity-gradient stabilized spacecraft is under consideration. The spacecraft librational motion is going under the action of the gravitational torque and the other torques of different nature providing disturbing influence in spacecraft oscillations. The perturbation technique is used for approximate analytical research of spacecraft oscillations. A special notation of differential equations of perturbed librational motion for gravity-gradient stabilized spacecraft is derived which is generalization of familiar equations in canonical variations, to the case when potential disturbing forces are operative along with non-potential disturbing forces, usable in a quadratic or cubic approximations. The derived equations proved to be convenient for the analysis of nonlinear spacecraft oscillations with the use of asymptotic methods of nonlinear mechanics and allowed to obtain new effects in nonlinear spacecraft oscillations.