I cannot adjust the brightness in game. Everything is as dark as a cave, and sliding the brightness bar does nothing. Using Windows 7. Any advice? Thx.

Is this with GLQuake (the Windows version that I believe is bundled in the GOG installer)? If so, then the broken brightness is an issue that has been around for a long time. I remember on my old Win98 machine when I played GLQuake that the brightness slider never worked and I had to up the brightness on my monitor whilst playing.

Nowadays, though, you are much better off using a source port, like QuakeSpasm (for a faithful Quake visual experience) or DarkPlaces (which allows high res textures and various lighting effects).

EDIT: Actually, the GOG version might be using 3DFX via nglide. If so, you can use the nglide configurator to up the gamma value.