This game is so dark I can't see when I go into a shadowed area. Problem is, upping the brightness changes nothing. It is literally exactly the same level of absolute dark no matter where I set the brightness slider. I'm stuck now in a basement on the Abandoned Building level, because I cannot see where I am supposed to go. How can I change the brightness in a way that will work, since the video menu slider does not? Or should I just consign this yet again to the "should not have bought" pile, with so very many other games? Damn shame to have to do that, since it looks like fun. Windows 7.

Two things. First, in buying this game on GOG you also paid for support by GOG for the game. Not sure why you would consign it without using support first? Second, brightness problem is likely an incompatible gamma setting between the different versions of nGlide as discussed here.

Thanks for the pointer. I'll give that a try tomorrow to see if it will help.

As for consigning it to the junk pile... I have 54 games in my account. 25 I've never played, because I bought them on sale and simply haven't found time yet. Of the rest, eight have both worked properly and been fun, a good investment of my time. Eight. I don't blame anyone when I just don't like a game, but a lot of the games I get here simply don't work, or have big glitches I have to work through. I don't have the time or patience for that. Half of what's in my account hasn't been touched for lack of time. If something doesn't work, I'm not going to spend a long time trying to figure out why. It only cost a couple of dollars, it isn't worth dumping so much effort into it. I'm here for leisure. I'm going to move on to what does work. I'll try the fix in the thread you pointed me toward, but if it doesn't work I'll delete Sin from my hard drive and find something that does. No big deal. Thanks again!

I had this issue with the steam version and found that you can just edit the config file
in the <install folder>/base/players\<playername>
eg for me it was SiN 1\base\players\blade
there is a config.cfg file if you search you can find:
set vid_gamma "<number>"

set the number lower to be brighter (just don't set it to 0).

vid_gamma did it for me, thanks.
a few (dark) areas of the game were almost impossible for me to navigate, and i couldn't see enemies unless they shot at me first.

in case anyone wants to change vid_gamma while in-game, then you will also need to use vid_restart afterwards, for the change to take effect.

you could also "cheat" and use r_fullbright 1, but it kills the atmosphere. (you can potentially type something like "bind p toggle r_fullbright" in console to directly switch between 0 and 1 by pressing P). this might be handy for that handful of super dark areas.

Sin Gold/Base/autoexec.cfg

//add below to existing file, or create file if doesn't exist, edit with notepad.

//Better brightness and contrast without washout
//intensity increases overall intensity of light, older video cards used variables as high as 2
intensity 1.2
//modulate seems to affect model brightness, older cards used high as 2.5, don't over do it.
gl_modulate 1.7
//you don't want your textures rounded down. Pathetic that this is never default off in any quake game.
gl_round_down 0
//mouse filter smooths mouse input over fps, useful for low DPI, removes jitter, personal preference
//remove slash lines to enable mouse filter
//m_filter 1
//I like my weapon switching this way
bind mwheeldown weapnext
bind mwheelup weapprev

//Beginning of Zoom Script
set my_sens $sensitivity
alias zoom_toggle zoom_in
alias zoom_in "fov 60;wait;fov 30;alias zoom_toggle zoom_out;set sensitivity 3"
alias zoom_out "fov 60;wait;fov 90;alias zoom_toggle zoom_in;set sensitivity $my_sens"
bind mouse2 zoom_toggle
//End of Zoom Script

//hand switch, hit to toggle modes
alias handy handy1
alias handy1 "hand 1; wait; alias handy handy0"
alias handy0 "hand 0; wait; alias handy handy1"
bind mouse3 handy

//fullbright switch, hit to toggle modes
alias fb_toggle fb
alias fb "r_fullbright 1; wait; alias fb_toggle fb0"
alias fb0 "r_fullbright 0; wait; alias fb_toggle fb"
bind "f" fb_toggle

//vid reset necessary for intensity changes, only changes after reset.
//bind v vid_reset
//enjoy the tweaks