[What is your typical Gamma setting?]

I have only changed settings for a few missions. Visibility of doors is a good gauge for darkness. 
Gamma: 1.2
Brightness: 0.8

[Free games]

Ashes series - 2063, Dead Man Walking, and Afterglow

Get the stand alone version in the files section.

Tips: Search for scrap in trash. Take the lantern battery and radio at the hubs.

https://www.moddb.com/mods/ashes-2063

https://forum.zdoom.org/viewtopic.php?t=47573

Other files:

Ashes Weapon Pack and Ashes Monster Pack - Use with other maps. Entering 'summon blursphere' in the console gives weapon upgrade kits.

Sterile versions - No story - Maps only - Use with gameplay mods. The first episode Sterile version is at the forum post linked above.

[The Complete Vengeance for a Thief Campaign]

The gear objective in part 2 did not complete in The Dark Mod 2.11. It worked in a 2.12 dev build, but I did not test other parts of the mission.