I am going to take a guess that the Zumo you got is the 660 and is the same as the BMW branded unit. You can plug these units straight into the GS wiring harness and all is finished.
On the GS, in front of the tank, below the head stock, on the left, below the wire bracket that holds the throttle cable (does all that make sense?) there is a plug that has been blanked off and tied into the wiring harness. This is the "Optional Accessory Socket". It has 3 wires going into it, one red with white trace, one blue with green trace and one brown. The Zumo will plug into this and work when the ignition is turned on and turn off 30 seconds after the ignition is turned off.
If you cant plug into this plug then it is best not to cut the wires as the 3rd wire (blue/green) is a sense wire that goes to the Central Electronics Unit and it should be connected. If your Zumo doesn't have the plug on it I can tell you what wires on the Zumo go to what colour on the bike.
If you do need to wire direct to the battery then when you unplug the Zumo from the craddle there should be no current draw and therfore no drain on your battery unless you have a short on the cable going to the craddle.
Hope this helps
An idea comes suddenly and in an intuitive way. But intuition is nothing but the outcome of earlier experience.