To be able to use gngeo-gui, you need to install gngeo, including the related bios. Once gngeo works, you can use gngeo-gui.
I picked up the file "gngeo-0.7-n900-binary.tar.gz" somewhere on this forum for that. The executable gngeo had some hardcoded paths in it which I hacked for myself to /opt/gngeo paths instead of /home/user/gngeo..
The emulator gngeo itself works rather well. Too bad the full screen mode is buggy. The keys where already set for use with the gamegripper. Make sure you also have a romrc.d map containing rom info.
Full information can be found on the original thread