See OSMC run add-on as root - #6 by dillthedog
On my Pi. default permissions for /dev/gpiomem are:
crw------- 1 root root 244, 0 Jul 22 19:43 /dev/gpiomem
If you add the udev rule, it will become:
crw-r----- 1 root osmc 244, 0 Jul 12 12:15 /dev/gpiomem