ISO setting changes typically result in relatively large exposure jumps compared to lens aperture setting changes. A lens iris can adjust in fractional steps (e.g.: f4.0 to f4.1 to f4.2, etc.) depending on the lens.
I'd recommend doing some tests first. You might get smoother exposure transitions if you lock your camera's ISO at the highest value required for the scene (don't use auto ISO).
Lock the GH2's shutter speed, too, typically at approx. x2 the frame rate (if shooting 24p use 1/50th, or if 30p use 1/60th).
Together with the above settings, use auto aperture (shutter priority) to adjust exposure.
Of course, some shots are challenging and some are "impossible". The GH2 has about 6-7 stops of dynamic range to work with. For example, compare that to the much more expensive Nikon D800 which has >11 stops of DR in video mode, or the new Blackmagic Cinema Camera which BmD says will have 13 stops of DR.
But no matter what camera is used, some scenes will be beyond its capabilities, and compromises -- or a different approach -- may be unavoidable.
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