Phase change materials (PCMs) provide a compelling platform for active nanophotonics. Among them, the PCMs, the chalcogenide glass Ge₂Sb₂Te₅ (GST) is particularly attractive for dynamic display applications, owing to its large refractive-index contrast and non-volatile phase-change behavior. In addition to this, elemental doping of GST is commonly employed to increase its thermal stability due to the increase in crystallization temperature. However, there exists a gentle phase transition (GPT) effect in doped GST that has not received adequate attention. The GPT effect transforms the abrupt phase transition (typically occurs as the annealing temperature increases) into a gradual process, which remains unexplored and could be the subject of further research in this field. Therefore in this work, we conduct a systematical study of optical GPT effect in Cr-doped GST and its related devices, exploring potential applications in high-precision structural-color control and non-contact temperature recording. The resistance-temperature (R-T) responses of Cr-doped GST reveal that the GPT effect can be promoted by higher Cr-doping concentration. The optical GPT effect of Cr-doped GST is validated by temperature dependent (n, k) of Cr_9.5(GST)_90.5 (CrGST) film. Based on the optical GPT effect, we fabricated the CrGST-based Fabry-Perot color manipulation device that exhibits a gentle color change around the phase transition temperature range. The color gamut and temperature sensitivity can be controlled by adjusting the geometric parameters of the device, and the colour saturation can be adjusted by changing the annealing temperature of the device. The CrGST device also possesses the potential of temperature recording. Based on the CrGST device, we designed a temperature-voltage conversion system to validate the principle of temperature recording. By adjusting the incident angle and laser wavelength, a large and linear V-T response can be acquired. We believe our work opens a new route in the area of high precision color control and non-contact temperature recording.