I think Arlong got captured by the Celestial Dragons. Arlong is one of the most racist characters in the series, murdering humans for spite, just like the CDs. He enslaved people and forced them to give him tribute, the same way the CDs do. Arlong is a dark mirror of the Celestial Dragons, showing how racism trickles down.
I believe Nami is a Celestial Dragon (and also Uranus and the Queen of the Moon, but that's for another time).
Nami was enslaved by Fishman who had been abused by the Celestial Dragons.
Nami was a Celestial Dragon abused by Fishman, but will free Arlong, the Fishman who abused her.
Nami already showed she was willing to forgive Jinbei. Nami has always shown a willingness to forgive children. I believe Nami will see past the adult Arlong who hurt her, and see the child Arlong who helplessly watched his people be enslaved and destroyed. Arlong suffered the same pain of slavery that Nami did, and caused Nami's suffering by mimicking the behavior of the Celestial Dragons. Hatred is a vicious cycle. I think Arlong is being saved for a big moment with Nami. Oda just turned Buggy into a Yonkou: the East Blue is about to have a comeback, baby.
If Gin actually shows up in this Manga again I will lose my fucking shit