If the map is too dark, add the command +gl_maplightmode to the .bat file and choose either 1 or 4. For example:
@echo off
start gzdoom.exe +gl_maplightmode 4
This way, the game will ALWAYS start with the pre-set gl_maplightmode command.
My After-DOOM gets too dark, so I need to use this command; mine looks like this:
@echo off
start gzdoom.exe +gl_maplightmode 1 -iwad DOOM2.wad -file After-Doom-v4.6.1.pk3 -file marifx_m-master.zip