Well, my Problem is, my map is lighted very good, on my computer, so when i start it as a lan game,. it looks very good.
When i upload it to our clanserver to test it with other people its much too bright, loosing much of its athmosphere.
I think its a gamma Problem, but how can i change it so, that it´ll use the gamma i intend to?

Or has the Problem another source?

Have you got an env_gamma in your map? If not, it will use the gamma from the last map, which might not be what you want.

i have an env_gamme in my map, but it doesn´t seem to work properly

It was confusing to me at first to realize that many native halflife.wad textures have special properties such as being lit by default.

Make sure you are not using textures that have a special character before them such as +, -, !, {, etc.

Your light entities should be checked to make sure they are not too bright.

Compile your map properly (full instead of fast) for the real end result of lighting.

Make sure the map properties specify a default light level of at least 25 not 0 (i heard this sometimes causes probs never experianced them)