Gungrave, Rainbow: Nisha Rokubou no Shichinin Recommendations
"Friendship" and brotherly bonds between the characters is a very strong theme in the beginning of both series....
Two great anime about friendship, it shows us the good side and the bad side of it. How can one be corrupted by money or blinded by love etc. Both story's have great emotional background, show's us how young people survive in war conditions. Also involves mafia and yakuza showdown. The main diference in this two anime's is that GunGrave has it's SciFi side, while Rainbow it's sport related side (boxing) to the story.
Gungrave is very similar to Rainbow in terms of atmosphere and content. Both series feature incredibly bleak and almost nihilistic settings, where brutal violence and terrible circumstances directly impacts the lives of the main characters. However, both stories have the central theme of brotherhood and striving for a future of happiness in a world that works against it, shining a small ray of hope through the darkest parts of events occurring. You're guaranteed to become emotionally involved in the things happening in both of these stories, hoping that everyone gets what they want and the story has a happy ending, somehow.
Both shows have a group of characters that started out in rough times. Both anime have friends that go against each other at times. Both shows have mafia involvement. If you liked Rainbow or any anime with that dark, gritty work your way up nature youll most likely enjoy Gungrave as well.