GH1-GH3 compared at ISO 200

Figured so. But that means the discrepancy between our results remains a bit mysterious. One additional (although perhaps far-fetched) possibility is that the GH3 plays around with RAW level amplification based on camera settings other than ISO alone. Using ExifTools to inspect the complete EXIF information in the GH3 and E-M5 RAWs I am comparing, I found a setting named GainControl, appearing just before Contrast, Saturation, and Sharpness, which in the GH3 files has the value "Low gain up" but in the E-M5 files has the value "None".

Is this set similarly in your case and do you or anyone else know what this setting is about? Apparently, it is not what Panasonic refers to as "Intelligent dynamic range control". I can see in the EXIF data that the parameter IntelligentD-range is set to off.