Gungrave is a video game based anime designed by Yasuhiro Nightow. The anime is directed by Toshiyuki Tsuru and developed by Red Entertainment. It centers around friendship, and betrayal, as two dudes, Harry MacDowell and Brandon Heat, try to climb their way to success in the mafia syndicate known as Millenion. Watching episode 1 (or not) determines whether or not Gungrave takes place in flashbacks. If episode 1 is skipped, Gungrave will come off starting at a "beginning" rather than going into flashbacks.
Here be the box art for both games:
I've yet to play either (not going to) so if you wanna know if they live up to the anime, you're gonna have to buy them on ebay for 5$ and find out for yourself.
Gungrave was originally made in 2003 based on the PS2 game. It aired on G4 in the AnimeUnleashed block since 2004.
As far as the rating goes, Gungrave has lots of violence, blood, profanity, and A LOT of people die. Thankfully there is no random sex scenes thrown in just for the sake of sex. This is a more mature anime and you won't be finding any sweatdrops, anger veins, or exaggerated facial expressions.
See more info in the Anime Bullet.