To start off, let me post what exactly Grave can do with his weaponry
(ignore Bunji [the one in the brown coat])
(watch till 2:25 for Grave's section to end)
Now to Grave's skill as a combatant. As Gin puts it, anyone worth a damn in the verse can simply predict their attacks, analyze their weaknesses, and more. This applies to Grave, because Gungrave is in the same verse as Trigun; set on the same planet, just in the past
To further quote Gin, I'm gonna copy paste their original post from the last match (so ignore the metioning of SlyFox); "Grave should scale above the likes of lower tiered Eye of Michael members (Note: Even these guys can tango with the likes of Vash albeit not for very long.) and should scale to the skill feats of the higher end members such as Livio and Wolfwood. The important part to note being that Livio has 2 layers of Analytical Prediction similarly to Slyfox's (Razlo his other
personality has his own unique form of precognition, Livio later learned how to control Razlo and synced up with him allowing him to keep pace with a
vastly faster opponent with their own
speed amps while using an
extremely unorthodox weapon. Said person also has an "
anti skill aura." that specifically hinders those more skilled than her and Livio still managed to beat her.)"
When referring to Grave when he was alive as Brandon Heat, one impressive skill he displayed was where after Bunji had been waiting with a sniper rifle to find a target without luck, Brandon walked in, instantly knew where the target was, took the rifle, and shot the target dead on from building away. This is impressive due to the fact that this target was being hidden by a curtain.
Not to mention Grave's versatility. He has regen that allows him to soak up bullets, rockets that can home in on the opponent, aoe with some of his attacks, perception manipulation that slows down his perception of time to a near halt, and the equivalent to built in lasers in his coffin as shown in the prior video to basically nuke the opponent.
Oh, and AP advantage
in Geneva of course 
