The Giant Steerable Science Mirror (GSSM) is the tertiary mirror of the Thirty Meter Telescope (TMT). To evaluate the performance of GSSM, normalized point source sensitivity (PSSn) is investigated. Calibration and metrology allow the estimation of telescope performance at different zenith angles. PSSn also realizes the prediction of the TMT main mirror assembly optical performance. The relationship between PSSn and slope root mean square (RMS) is analyzed theoretically when evaluating the performance of GSSM. First and foremost, the pointing performance of the GSSM prototype (GSSMP) is specified by PSSn and calibrated by a laser tracker. Then, the tracking performance influence on PSSn is taken into consideration. The jitter of the GSSMP also contributes to the degradation of PSSn, and it is also discussed. Lastly, the interaction between GSSM and the main mirror unit is also revealed by PSSn.