First I would contact the GPS company and ask if they have a disabling feature on this system and if they can provide the installation manual for that unit.
If no success there, disconnect positive post at the battery and install a test light or amp meter between the cable and post.
Disconnect or pull fuse's or connectors thought to go that componet (1 at a time) and watch light or meter for a drop in amperage drawn. Please note that the door switch must be held closed. There is going to be a minmum draw to keep radio and ecm memory but they are less then.0487 amperage most of the time. and will not light a 12 volt test light.
Test drive and find out it performs alright.