It's not quite like Singer where you can just go to a compiled list, unfortunately. Many of us are attempting to gather info but for now, here are a couple good places to start. (Gotta love Google Search)
Also I few picture would really help.
http://www.sewmuse.co.uk/german%20sewing%20machines%202.htm
http://www.sewmuse.co.uk/Gritzner.pdf
http://www.sewalot.com/gritzner_sewing_machines.htm
http://fiddlebase.jimdo.com/german-machines/gritzner/