Not sure of the wiring or accessory circuit on your bike.. But, for a dedicated GPS power source you could use a relay, which is basically a switch that turns power to the circuit on or off, when you turn your bike on and off.. The relay itself is cheap - about 3-5 bucks.
Connection 30 - Run power to your relay directly from the battery.
Connection 86 - Tap into a wire on your bike which runs hot only when the bike's key is turned to the on position (like a headlight wire)
Connection 85 - To any ground source on your bike
Connection 87 - This is your positive lead which feeds to your accessory fuse-block, or directly to your GPS
You don't need to wire the switch inline on Connection 86 unless you want the ability to turn power off to your GPS when your bike is running.
If you get a cheap multi-tester you can soon find a wire which runs hot only when your bike is turned on for Connection 86. Relays don't use any real amount of current to operate their switch, so, wouldn't overload any existing circuit by tapping into it for Connection 86 (that I am aware of at least). But my favorite wire to tap into, to trigger a relay's switch, is the headlight circuit, as there is usually no guessing to the current headroom or interference for doing so.
All this may be moot, as you may already have an accessory circuit that you can wire directly to, I would check your owner's or service manual.
If you feel you will want to add more accessories later on - you can buy and mount an additional fuse-block and run it off your accessory circuit or relayed power source.
The good thing about using an accessory circuit, or relayed power fed directly from the battery, is that you don't overload any other circuit on your bike.. Which would be bad.. Sometimes real bad.