I'm using this converter with GH3:
http://www.kopterworx.com/Osd-/-fpv-/-hdmi/HDMI-to-AV-converter-mini?keyword=hdmi
Yes, it's pricey.. But, it works without external power, just connect the camera and 5.8GHz TX. It comes with a nice quality HDMI cable, but you have to mount the converter to...