The map was made before HDR was added into the game.
I do not have the map editing skills or time to remove the boosted HDR from the map :(
If anyone with the time and skills required can then please do so, Just remember to give credit XD

For now though, If the map is too bright for you just do this simple fix:

In Gmod go to options, Video, Advanced,
Then set "High Dynamic Range" to None.

Hope this helps!