What happens on your pc is the normal behavior of windows, when there are videos playing the standby is never activated. One solution is to install the MonitorOnOff plugin and configure it so that the PC is put into standby after the specified time.
On Windows 10 and 11 when the monitor is turned off by power management Windows kills the screen saver so gPhotoShow will never have a chance to turn on monitor again. The same happens for system standby. The only solution is to run gPhotoShow Pro as slideshow instead of using it as screen saver