The arena has an open roof. I know it. I once jumped over the whole thing.
__
BTW: This "brightness control" solution is ... uhm, imperfect ... this is not "gamma correction", it is rather "changing the crystal brightness", and does only work in maps where the "crystal light" has a specific name, I guess. E.g. only at the Hydlaa plaza, but not in Kada-El's tavern or in guild houses, especially not in the Death Realm.
And it doesn't affect the GUI.