Getting into the show itself, the main character is the tall, dark and lanky Van, who travels the world with a single purpose in mind: To kill the mysterious clawed man who took his wife, Elena, from him on his wedding day. Van isn't exactly your average protagonist, as he is quite apathetic, snarky and not the most sane, with him perfectly willing to let the starter village from the first episode burn since it doesn't concern him at the start. Most of his heroics come from other characters baiting him, being on the job for money or the fact that he DOES have a code of ethics which he adheres to to an extent: Slipping out of that, of course, is when he's at his worst. Van is also rather interesting in that the show doesn't take a generic "vengeance is bad" route and shows it in a far more fascinating manner, and Van's contrast between his normal self and when anything regarding The Claw is near are a highlight of the show.
He is primarily joined by Wendy, the gun to Van's sword, a girl from said village who travels with Van in pursuit of her brother, who has been kidnapped by the very same clawed man. Compared to Van, she is much more optimistic and enthusiastic, but she isn't an overeager kid archetype either and is still rather serious. The two are joined by various characters throughout their journey, but going too far into it would be somewhat spoilers, and many of them do not stay for as long until closer to the end, but they end up for the most part good.
The Claw, naturally, is the main villain of the piece and he is a pretty amazing piece of work: In some ways, he feels like a bit of a deconstruction or different look at the idea of a "sympathetic" antagonist, as many of his actions + his personality are somewhat incongruous and his personality is so far removed from the reality of the situation that he comes across as far more insane than most "crazy" villains you will see in media. It also makes him rather hilarious in many ways and the reveal of who he is has to be seen to be believed. He ends up with a fun and crazy cast of minions and underlings as well, but I will say one of the minor issues with this is that his minions don't all get the development they could use, although the ones that do are pretty great, but there is clear missed potential there.
Although the story starts off with a good deal of one shots, most of them are actually quite good, and the show actually goes on to make almost all of them relevant in some manner, some small and some large, leaving the show with shockingly little filler to its name, and the one shots are almost all enjoyable anyway, many of which with some pretty out there concepts. The show especially picks up after Episode 5, which introduces the rival character Ray Lundgren, who is...a rather hilarious rival archetype really, as his primary rivalry with Van is who will get the last hit on The Claw, as Ray ALSO desires vengeance on The Claw.
Artistically, the show reminds me of old school anime in a lot of respects, and it does have some really good shots at points, but the art can sometimes get a bit underdetailed and I feel like the mecha fights while usually good aren't always the best or most well done. The art is at its best when doing stuff like Van's expressions or the cockpits of the mecha. Musically, though, it is a lot better, with the opening being pretty epic and the ending especially being memorable. The music that plays when Van summons his mecha, Dann, is very nice and it has a smooth and noticeable take throughout.
Overall, Gun x Sword is a pretty good anime with some fascinating takes on the concept of vengeance, especially refreshing in that we don't get a generic "vengeance is bad" aesop, a pretty great hero and villain, plenty of good comedy with a good deal of nice action and a solid if sometimes issued story, and a lot of memorable characters. If you're not convinced yet, I'll leave you all with just three words: Mexican Power Rangers.
