The bug where GDMs don't fire can still happen sometimes (it was said to have been fixed in the last update). I attacked a planet using Undines and all the GDMs got destroyed on the ground even though they're supposed to have the same range as Undines and should fire when they come in range.

GDMs are not always seen to fire from the planet attack screen, although they can be seen to cause casualties in attacks where they fire successfully.

GDMs seem generally ineffectual in this version of Anacreon. I have never seen players intentionally build them. They're single-use, have the same attrition as other defenses, and are completely interceptable by relatively small jumpship fleets even in huge numbers.

Perhaps player-built GDMs should have reduced or no attrition to offset their single-use nature?

GDMs built by independent worlds shouldn't accumulate excessively since newly-arriving players still need to be able to conquer independent worlds late into the game.