"I have been trying to interpret simple, but am having loads of trouble with GND (Ground, right?), now is this just the returned path to the batteries +V , or dose it need to be attached to the case? "
Yes, your first impression is correct, it is just the return current path to the voltage source whether battery or DC power module. A better term in my opinion is 'common' or 'circuit common' as 'ground' tends to imply a connection to an earth ground somewhere and that is not necessarily true or even required. Having and using a true earth ground is usually done as a safety design for voltages that are high enough to cause injury. An earth ground can also help improve radio frequency transmission and reception but that is a whole other subject.
Lefty