Nope, you cannot change the gamma in the game...well it "can" be adjusted but it doesn't have much if any effect on the game. Even with the slider all the way up the game is way to dark.
My suggestion is to go into the graphics option and change lighting from Lightmap to Vertex. Although this does not look as good, atleast levels will be brighter.
Other than that I guess you have to crank up the brightness control on your monitor.
Take care, and goodluck!
JrPean
P.S. You will also notice that changing the Gamma in your Display Properties in Win2000 will have no effect either...obviously a driver problem here...hopefully there will be a fix soon!