If you get consistantly good exposures metering with the Gossen and the various cameras, then there may be a problem somewhere in your technique, although what you describe seems ok.
With the Gossen, how close are you to the grey card? If I were doing that test I'd be no more than 3 or 4 inches, FWIW.
What happens if you make a reflected reading with the Gossen, then rotate 180 and make a reading with the incident dome? The two should match.
Also, with the cameras, are you focusing on the card or working with the lenses set to infinity? Setting them to infinity would be preferable.
Finally, if you scan around metering various portions of the card, do you get any difference in the readings with any one of the meters? I.E. are you getting any bright areas of light on the card that may not be easily visible?