Abstract: We have studied the profile of potential modulation introduced to a GaAs/AlGaAs heterostructure two-dimensional electron gas (2DEG) by a striped gold gate with period a=500nm on the surface. The magnetoresistance oscillation of 2DEG under one-dimensional periodic potential modulation arising from commensurability between the period and cyclotron diameter has been measured as a function of bias voltage applied to the gold gate. The bias alters both the amplitude and lineshape of the potential seen by 2DEG, with concomitant change in the magnetoresistance oscillation. The analysis of the magnetoresistance shows that with a positive bias, the modulation amplitude is considerably reduced and the third harmonic component becomes conspicuous. This suggests that the "built-in" potential modulation, caused mainly by strain resulting from differential contraction between the gold and GaAs, has more rectangular landscape, than the potential added by the bias. The strain-induced third harmonic component accounts for the previously unexplained extra dips in the magnetoresistance traces for a magnetically modulated 2DEG with a ferromagnetic metal gate.