Do a search on my username, Vampire, and you will see a number of entries on "Givi top box stop lights" and the Touratech TPS 15 power module.
In brief, there are a number of successful approaches, but I found that wiring up a relay is the safest. Others have reported successfully taking power directly from the bike's stop light, so that is certainly worth a go as the simplest solution. The worst that will happen is that the GS's circuitry will complain, but once you disconnect your additional wiring the system resets itself.
Can be made to work.
Good luck.