The wiki article is interesting.
I find it unlikely that Akai had come into close contact with Gin prior to his promotion, since as the article says, they'd probably not have had to send the scout. Yet it's stated in the manga that Akai used Shiho to climb the org's ladder, and with the extreme likelihood that Shiho was in some kind of relationship with Gin, the two must have bumped into each other from time to time. So could it be that the Org smelt something fishy going on and wanted to make sure (hence the extra caution)?
Still, I don't see how Akai can be so hateful towards Gin. He can't blame Gin for Akemi's murder, since he's just following an order (he should save that for the boss), and he probably doesn't even knows that Gin's the one doing the deed. So I guess whatever causes their antipathies must have happened during the 2 years when the BO tried to take Akai's life. I hope Gosho'll shed more light on it, but it seems he'll focus on the Akai/Bourbon showdown first, which means we'll probably have to wait 3-4 more years to have the answer.