[[WMG: GunGrave is the earth from the Trigun Universe in the Past!]]
Here are my point for this:
1. There were some references to Trigun in the Gungrave anime and Videogame, but one of them was the existence of the Chicken creatures that they use for transport in Gunsmoke appearing as racing animals in GunGrave. Which suggests genetic engeneering taking place in that world.
2. The Seeds. In Gungrave OD the group uses some sort of Seed Creature to t rack seed, this creature is kept in a bulb shaped glass (Just like the Plants in Trigun) and it had some sort of wings (again just liek the plants).
Now Where do Plants come from? SEEDS! Which was the name of the colonizing project? SEEDS!
My Theory is that the Seed was the key element in the development of the Plants, and Gungrave 3 will narrate the evenets that led to the Earth needing to develop the Plants and colonize another Planet.
* In the anime, it seems possible that Gungrave takes place in the future of Trigun, since Big Daddy told Brandon that the city ''used'' to be a desert, like the world of Trigun, plus the overall technology is more advanced than that of Trigun. The presence of the racing animals also implies that it's the same world.

[[WMG: Gungrave is an alternate universe version of [[RyuGaGotoku Yakuza]].]]
Spoilers for both series, naturally.

The main character of Kazuma Kiryu is a spectacular fighter who quickly rises up through the ranks of a mob family, until that is ground to a screeching halt because of his loyalty to his best friend who has killed someone (... in Brandon's case he is the one who is killed, in Kazuma's it's someone else that Brandon takes the heat for it). They are both, despite their professions, absurdly nice men.

When both of them return (stronger than ever), they both wind up protecting the daughter of their love interest, who after their relationship couldn't work, went on to be involved with a very powerful man and have his child. In the end they wind up fighting their old friend but, at the last minute, they reconcile somewhat (though the friend still dies).

[[WMG: The reason why Brandon/Grave gets less direct focus/development than the others?]]
'''Note:''' Applies primarily to the anime, since the games take a different tack and OD introduces two other main characters.

It's from Brandon/Grave's point-of-view, so he becomes the AudienceSurrogate: You're seeing everything through his eyes. Brandon's defining feature is how he places others before himself, and his lack of self-interest is what puts so much of the spotlight, and subsequent development, on other characters. The ones Brandon has more invested in, as a result, get more development and screen-time because Brandon affords them more personal regard.

[[WMG: Harry wasn't planning on killing Big Daddy or Brandon until ''after'' Brandon decked him.]]
Though not thrilled about being passed over, Harry didn't seem particularly disturbed by Big Daddy's assessment; given how quickly the heir wound up with an "accident," Harry seemed to have other plans in place. Brandon's decision to honor Big Daddy's ideals put him in conflict with Harry, who of course was secretly a traitor: The conversation in the elevator was intended to test Brandon's loyalty, and his response told Harry that Brandon had chosen Millenion and Big Daddy over Harry and his vision. What followed was unplanned, but what was done was done; knowing that Big Daddy would take an interest in Brandon's death, Harry had no choice but to go ahead with what had previously been just bait, leading into the spiral of events that culminated in Bloody Harry [=MacDowell=].