I don’t think gamma is having an impact on skins looking too bright or too dark in the game. The only things gamma and monitor/video card settings are going to change is the colors. What one person sees as red on their monitor may look pink or purple to someone else. When making skins, you have to balance the colors used on the skin – light green vs. dark green for example – with the setting in the material.txt file so the skin looks "natural" when viewed in the game. (And check your skins at night because I have seen a skin in the game that actually glows in the dark!) If the gamma impacts the skins and the terrains the same, then a skin that looks "natural" in brightness at one gamma setting should look "natural" at all gamma settings.
When I was making skins, I took my cues from the terrain tiles to determine sun strength and the amount of light reflection to set the skin’s brightness. With these new darker tiles, the terrain looks as if it is overcast or cloudy – no sun shine – in the immediate area but sunny in the distance. I know with my skins they don’t look "natural" any longer because they look like they are in full sun with too much light reflecting up on the undersides. Changing the gamma won’t – or shouldn’t – make a difference.
What you have to do is come up with a common definition for "natural." And with this community, that shouldn’t be any problem at all.
