Linedef lighting problems

This is a feature of the game, making perfectly vertical or perfectly horizontal lines slightly brighter or dimmer.  The idea is to make the shape of rooms easier to see.  If you are targeting a port that supports MAPINFO, you can add evenlighting to the entry for your map.  Otherwise you are stuck with it.  There is one other workaround: Move one end of the line 1 unit, so the line is no longer perfectly vertical or horizontal.

What are your screen settings?

I increased the gamma and got this:

@Kappes Buur Whatever the defaults are for GZDooM. And thank you @Empyre, smoothlighting works.