Hey,
the first thing I want to mention is what can be found in this and other threads: The GPIO bindings doesn’t seem stable and often not working as expected. I would recommend to not use it. If you want to interact with gpios, an alternative approach is an external application and the exec binding.
With openHABian (so more precisely with a minimal Raspbian) there is no gpio group. What would you need it for? Last time I checked the gpio pins belong to root. Please check openHABian hassle-free openHAB Setup
You will need to do some testing, I can only help as much.