The use of a GaP barrier in a vertical-cavity surface-emitting laser (VCSEL) and its effect on output power at 860 nm were investigated. The GaP barrier was found to have higher conductivity from current-voltage measurements. This material is suitable for its use as a current spreading layer in the VCSEL based on (Al)GaAs. Experimental measurements show that the output power from the VCSEL effectively increased as the thickness of the GaP barrier increased, eventually saturating for a specific thickness value. The calculated real emission area and optical properties show that the current spreading effect in the GaP barrier is limited by the current window. Among different VCSEL chips, the highest output power of 26.5 mW was obtained using an optimal 3-μm-thick GaP barrier, which is an increase of 43% compared with the power output from the conventional structure. Nearly equal output powers were observed for structures with 3- and 6-μm GaP barriers. The results show that the GaP barrier used as the current spreading layer is essential for improving the output power from the VCSEL at 860 nm. The optimal thickness of the GaP barrier in the VCSEL strongly depends on the aperture diameter (real emission area), which can be determined from the current window.