One issue with using a GH3 camera inside of the Gearbox GB-2 is trying to access the battery door. By using the Panasonic DC Coupler (dummy battery), i'm able to use an external power source like the Tekkeon MP3450i Battery (found here). The Tekkeon battery is capable of outputting various DC Voltage including the necessary 8.4V DC to power the GH3 camera. This allows me to use the the GH3 in this Video Cage for longer run times. Links to the equipment in this video found below.
Panasonic GH3 DC Battery Coupler External Battery
Tekkeon Battery Packs + Tekkeon Extended Bundle
Panasonic GH3 Digital Camera
P&C GearBox GB-2 DSLR Video Accessory Bracket