Thank you for your suggestion. However, there is no .ini file in the game folder.
There is the dosboxWC3.conf file, where I can set:
# output -- What to use for output:
Changing from Ddraw to overlay, opengl, surface seem to change nothing (except for mega-lag when set to surface).
It's as if the brightness had been turned up a LOT. If I load earlier missions, then it's fine, however.
I have also attempted the ddraw hack by Jari Komppa (2010), but placing ddraw.dll into my game folder does nothing, possibly because the ddraw hack was designed for the Windows version, not the DOS-emulated one (there is no .exe file in the GoG release per se).
I found another GoG thread for the same issue (finally): https://www.gog.com/forum/wing_commander_series/on_certain_missions_my_gamma_settings_are_way_off_everything_is_too_bright_and_w
It is indeed for a mission set in a nebula. Perhaps it was an intended effect. It just looks ... bad.
However, I can share something that I'd fixed earlier: I was having problems with cutscenes (in WC3) "pausing" before transitions, that was resolved by launching the WC3 Launcher Config (LaunchSettings.exe) and then, when there's an automatic test for SVGA, there's a line of text at the bottom saying "if you see this press Y" ... and after pressing "Y", the problem was resolved and cutscenes played as normal (it switched to alternative SVGA or something like that).
Hope that is helpful to others as well.