As long as your factory heated grip lead isn't being used, you an use this: Eastern Beaver Heated Grip Adapter
Cut the power cable for the GPS, connect ground to ground, power to power, and plug into the heated grip lead on the Strom. The only issue is the heated grip lead on the Strom is switched, so if you turn off the bike you'll lose power to the GPS.
Other than that, your options are:
1) A direct run back to the battery, with fuse.
2) Purchase a fuse block (for your application, the Plug and Play Complete 3CS VStrom Kit may work best)...get the one with one unswitched lead, as this will allow you to power the GPS when the bike is off).
3) Purchase a power outlet and use that. You can use the automotive cigar outlet, or for more vibration resistance, you can purchase the Powerlet. As your GPS probably did not come with a Powerlet adapter, you'd have to purchase one, or buy the male Powerlet lead and install that on the GPS power cable.
It's funny how Eastern Beaver has a solution to whatever path you want to take. Jim makes excellent stuff.