Gig Performer 1.8.2 includes an 8 track audio file player that can be used for various purposes including playing backing tracks.
If you want to play your backing tracks independent of your main rig rackspace setup so that you can switch between rackspaces freely and not interrupt the backing track - simply create a new instance of GP just for this purpose.
Click on File->Create New Instance and name it something like “backing-tracks”. A new GP will open where you can create your rackspace, drop your Audio File Players and load your backing tracks.
You now have a completely independent backing track system that can be controlled with your MIDI or OSC devices and you can recall it any time by selecting "Open Existing Instance from the main File menu.