Gilgamesh is one of those shows that you don't expect much from and end up being presently surprised at how much you enjoyed it. It does however require some patience as it starts off pretty slowly and maintains a slow pace throughout.
What I liked
One of the main things I liked about the show was how it seemed so generic in the first 2 episodes (its teenagers with superpowers fighting other super-powered teenagers) but then managed to do some unique and interesting things. First off while it does seem like your standard good vs evil story , the characters themselves don't really fit into the good-guy/bad-guy roles. Isamu, Fuko and Tohru who are the "good-guys" are not your altruistic goody two shoes heroes who are trying to save the world. Their main reason for fighting Gilgamesh is that they don't have anything better to do. Initially they don't seem to really care about anything , Isamu in particular is a very stoic apathetic character, but as the series progresses they develop nicely into quite likable characters. The countess who leads this group also does not fall into the good-guy character and in many scenes is an out-and-out bitch. Her reasons for using the children to fight Gilgamesh are entirely self-serving and she does not care about saving the world. The countess is by far the most interesting character here as she can come across as a cold hearted bitch in one scene and later on we see her as a lonely woman who is a slave to her past and at times you feel sympathy and compassion for her. Likewise those on the side of Gilgamesh, while technically villains, are not what you would call evil and in their own way are trying to save humanity. The art style is also really good. The world of Gilgamesh with its sherltering sky and such is not a happy place and the art style really gets that point across with a load of dark colors and shades and everything has a real goth feel to it. And lastly while the plot moves at quite a slow pace there is a good story here and a quite interesting ending.
What I didn't like
Alas not all is good with this show. There are some real flaws and one big one in particular. That main flaw being that while at its core this is a good vs evil story very little time is spent developing the characters on the Gilgamesh side. Other than appearing in battles we never get to see them and the main villain, Enkidu, does not even show up until very late in the series. This was such a missed opportunity as some of the members of Gilgamesh look really badass and from the glimpses of their personality they seem really interesting characters. The show would really have benefited from spending some more time developing the members of Gilgamesh.
Another thing I did not like was the battles and action scenes. They are not bad per say just very generic and not that fun to watch.
Overall I liked Gilgamesh. It is not going to break any new ground and its overly dark, depressing tone and art style can be off-putting at times and the pacing of the story could have been better. But while it starts off slowly it steadily builds steam and finishes well , and it has some really interesting characters and relationships.