So you want to make the map darker well do I have the addons for you!

First the addon storm fox will make the map dark if you set the time to night

How to set it to night https://imgur.com/a/vlNEzfi

What it will look like https://imgur.com/a/qIeguVo

Storm Fox https://steamcommunity.com/sharedfiles/filedetails/?id=1132466603

Second if you want the map even darker you can use the gmod fog editor. Its located in the entities tab

How to edit and use the block https://imgur.com/a/aSDAyrI

What it will look like https://imgur.com/a/NG3yBg9

If you have any questions just ask! hope I was some help to the people who wanted this map darker!