How to install my 'pi desktop debian package'

That file is possibly ok but is built for Debian stretch.

The correct file for Debian jessie is http://archive.raspberrypi.org/debian/pool/main/r/rpi.gpio/python-rpi.gpio_0.6.3~jessie-1_armhf.deb

The package comes as standard with Raspbian, but is not available in the Debian repo that OSMC uses.