Just above that post there’s a link to OSMC run add-on as root - #6 by dillthedog where I suggested a rules I called /etc/udev/rules.d/01-gpio-permissions.rules containing the line:
KERNEL=="gpiomem", SUBSYSTEM=="bcm2835-gpiomem", MODE="0640", GROUP="osmc"
This udev rule can be found on my Pi, which was a complete reinstall on 10 June, though strangely dpkg can’t find its source package. Can you find /etc/udev/rules.d/01-gpio-permissions.rules on your machine?
As I recall, I used udevadm to find the correct KERNEL and SUBSYSTEM names for /dev/gpiomem:
osmc@osmc:~$ udevadm info -a /dev/gpiomem
Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.
looking at device '/devices/virtual/bcm2835-gpiomem/gpiomem':
KERNEL=="gpiomem"
SUBSYSTEM=="bcm2835-gpiomem"
DRIVER==""
If you read the rest of Troubles with reading my GPIO device - #5 by sam_nazarko you’ll see that @sam_nazarko’s proposed 996-fix-gpiomem.rules had exactly the same error, so I’m not sure why it made into the OSMC build six months later.