The battery could be discharging because there's a problem with the bike's wiring that is causing a constant load on the battery, even when the bike is not running. This can happen when accessories are added directly to the battery versus through a switched circuit (i.e. the ignition). It could also be discharging simply because it's old and worn out (especially since you've fully discharged it at least a few times now, which is not healthy for a lead acid battery like yours).
You need to determine which of the above two scenarios is the problem, because the solution will be different. If you can get the bike to a shop, they can test the battery's condition for you. Or, you can completely unhook the battery from the bike and let it sit for several days, and see if it discharges "on it's own" with nothing connected. If it does, and it won't hold a charge well enough to run your bike after sitting, you should replace the battery.
If the battery is totally fine after sitting unconnected to anything, you probably have a parasitic drain somewhere on the bike that's drawing the battery down. Since you mentioned the bike has an aftermarket GPS tracker and alarm, that's probably a good place to start. Double check that it is wired according to it's instructions. Or, as a process of elimination, you can remove it and see if the problem goes away. If the problem remains, you can begin working through the rest of the machine's wiring. There are lots of resources on the web, which you can find with some googling, to help you track down your parasitic drain.
