In this paper, the modified linearized Phillips-Heffron model is utilized to theoretically analyze asingle-machine infinite-bus (SMIB) installed with SSSC. Then, the results of this analysis are used forassessing the potential of an SSSC supplementary controller to improve the dynamic stability of apower system. This is carried out by measuring the electromechanical controllability through singularvalue decomposition (SVD) analysis. This controller is tuned for simultaneously shifting theundamped electromechanical modes to a prespecifed area in the s-plane. The issue of designing arobustly SSSC- based controller is considered and formulated as an optimization problem accordingto the eigenvalue-based multi-objective function consisting of the damping ratio of the undampedelectromechanical modes and the damping factor. Next, considering its high capability to find themost optimistic results, the Gravitational Search Algorithm (GSA) is used to solve this optimizationproblem. Wide ranges of operating conditions are considered in design process of the proposeddamping controller in order to guarantee its robustness. The effectiveness of the proposed controlleris demonstrated through eigenvalue analysis, controllability measure, nonlinear time-domainsimulation and some performance indices studies. The results show that the tuned GSA based SSSCcontroller which is designed by using the proposed multi-objective function has an outstandingcapability in damping power system low frequency oscillations, also it significantly improves thepower systems dynamic stability. Manuscript profile