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In this study, the photoelectric properties of a complete series of GaS_1−<i>x</i>Se<i>_x</i> (0 ≤ <i>x</i> ≤ 1) layered crystals are investigated. The photoconductivity spectra indicate a decreasing bandgap of GaS_1−<i>x</i>Se<i>_x</i> as the Se composition <i>x</i> increases. Time-resolved photocurrent measurements reveal a significant improvement in the response of GaS_1−<i>x</i>Se<i>_x</i> to light with increasing <i>x</i>. Frequency-dependent photocurrent measurements demonstrate that both pure GaS crystals and GaS_1−<i>x</i>Se<i>_x</i> ternary alloy crystals exhibit a rapid decrease in photocurrents with increasing illumination frequency. Crystals with lower <i>x</i> exhibit a faster decrease in photocurrent. However, pure GaSe crystal maintains its photocurrent significantly even at high frequencies. Measurements for laser-power-dependent photoresponsivity and bias-voltage-dependent photoresponsivity also indicate an increase in the photoresponsivity of GaS_1−<i>x</i>Se<i>_x</i> as <i>x</i> increases. Overall, the photoresponsive performance of GaS_1−<i>x</i>Se<i>_x</i> is enhanced with increasing <i>x</i>, and pure GaSe exhibits the best performance. This result contradicts the findings of previous reports. Additionally, the inverse trends between bandgap and photoresponsivity with increasing <i>x</i> suggest that GaS_1−<i>x</i>Se<i>_x</i>-based photodetectors could potentially offer a high response and wavelength-selectivity for UV and visible light detection. Thus, this work provides novel insights into the photoelectric characteristics of GaS_1−<i>x</i>Se<i>_x</i> layered crystals and highlights their potential for optoelectronic applications.