Guess I'm a little way too late to leave a review but, anyway.
Basically Gungrave deserves more love. It's not exactly the most thrilling mafia action you might be expecting, but it'd be very much unfair to say it disappoints.
Mafia story? Check.
Strong bonding and relationships? Check.
Sci-fi? Check.
Pretty boy? Check.
Bonus point if you've played the video game before. (personally I was shouting DEMOLITION SHOT, DEMOLITION SHOT in the heat of the battle scene)
you wanna enjoy Gungrave to bits? Get into Trigun first. When you're a fan of Nightow-sensei's work, you'll be looking for parallels in his other works as well. Which are, honestly, plenty present in Gungrave and that's what I loved the most about the whole thing.
Additionally if you're looking for something a little tearjerker-ish, you're welcome to watch it(which I consider a little tribute to Trigun.)